I decided to knock off a bucket-list item and attend my first US Open this year. I have not bought any tickets but i'm hoping to go at least 1 day, maybe 2. Are there any recommendations where to buy reasonable priced tickets? Should I just pre-buy them through the US Open website now? If i plan to attend 1st wknd match, I noticed that the 7pm matches are cheaper. Is this the way to go if i'm on a bit of a budget? I'm also planning to stay in Manhattan. Is it an issue catching subway late at night if matches run super late?
1) The best way to buy single session tickets for the U.S. Open is to buy them when they go on sale in early June (or during the American Express pre-sale that occurs the week before that, usually starting the day after Memorial Day). Failing that, the next best option, if direct sale tickets from the USTA aren't available at Ticketmaster or at the box office (and they won't be on the Saturday through Monday of Labor Day weekend or probably the Friday of that weekend either) is to buy resale tickets the night before or the morning of the day you attend when many people are selling their tickets and it (usually) becomes a buyer's market for tickets. Ticketmaster is the official ticket partner of the U.S. Open, so I recommend buying tickets there, but other resale sites like StubHub, SeatGeek, etc. should also have tickets available for sale.
I'll note even direct sale and night before/morning of resale (day session) tickets won't be cheap relatively-speaking; if you buy Ashe day session tickets they will still likely be in the $100 to $150 range for 3rd/top level seats (I think they are called Promenade). On the other hand, you potentially get 12 hours of tennis if you stay the entire day, so if you look at it that way the cost isn't really that high. I would recommend trying to buy 2nd/middle level seats in Ashe if possible, if the cost isn't dramatically more than the 3rd level seats.
2) If you are on a budget, your best option is to attend the 1st or 2nd round (first Monday through first Thursday). On most/all of those days except maybe the Thursday, you probably will be able to buy tickets at the box office the day of play. Day session tickets cost more but provide a lot more tennis. You can enter the grounds before play starts at 11 AM with a day session ticket but you can't enter until 6 PM with a night session ticket.
An even better option with a budget is to attend qualifying the week before the main draw starts. Qualifying is spread out over (I think) four days, Tuesday to Friday. Entry to the grounds is FREE during qualifying.
One more note on the different types of tickets - there are four different types of day session tickets and two different types of night session tickets:
DAY SESSION TICKETS (entry as early as 9:30 AM before beginning of play at 11 AM)
*Ashe Reserved: provides reserved seat in retractable roof Ashe Stadium for the day session and access to all unreserved seats on all other courts the entire day (all courts besides Ashe have at least some unreserved seats)
*Armstrong Reserved: provides lower level reserved seat in retractable roof Armstrong Stadium and access to all unreserved seats on all courts except Ashe (no Ashe access) all day; this includes Armstrong unreserved seats during the night session
*Grandstand Reserved: provides courtside/near courtside reserved seat in non-roofed Grandstand and access to all unreserved seats on all courts except Ashe (no Ashe access) all day
*Grounds Pass: provides access to all unreserved seats on all courts except Ashe (no Ashe access) all day
NIGHT SESSION TICKETS (entry at 6 PM or later)
*Ashe Reserved: provides reserved seat in retractable roof Ashe Stadium for the night session and access to all unreserved seats on all other courts for any matches on-going/still scheduled after entry
*Armstrong Reserved: provides lower level reserved seat in retractable roof Armstrong Stadium and access to all unreserved seats on all courts except Ashe for any matches on-going/still scheduled after entry (no Ashe access)
One of my earlier posts breaks down the amount of reserved vs unreserved seating on all courts; take a look at that post for more details.
3) You'll have no issues getting the 7 train back to Manhattan after you leave/after the end of play. After the last match of the night is complete, the NYC MTA will hold a 7 train and run it super express back to Manhattan (only 3 or so stops before you get to Manhattan). Many other people who also attended the U.S. Open will be on that train with you. Even if you don't get that super express 7 train, the NYC MTA subways operate 24/7, so you can still get back to Manhattan without a problem.
I do have a couple of recommendations related to your last question:
*For even faster (though slightly more expensive) travel between Manhattan and the USTA NTC, consider using the LIRR Port Washington Branch. The trip is about half the time relative to a local 7 train (which is what you may be using to get to the USTA NTC). The LIRR Port Washington Branch stops (starts) at Penn Station in midtown Manhattan. (By contrast, the 7 train stops slightly further north, at Grand Central Terminal and Times Square among other stops.) The LIRR platform is also closer to the USTA NTC than the 7 train platform is. (The 7 train platform is closer to Citi Field where the Mets play.)
*To save money on lodging, you could try to stay at a hotel near a 7 train station in Long Island City in western Queens across the East River from Manhattan. The two years I attended the U.S. Open for multiple days by myself (2015 & 2016), I stayed at a hotel near the Queensboro Plaza 7 train station. That worked great; not only was the hotel near a 7 train stop, but Queensboro Plaza is an express stop, so that super express train I mentioned above stops there. You aren't going to get a 5 star hotel in Long Island City, but you will get something solid. Many, many fans attending the USO for multiple days stay in this area.