Tennis shoes seem to last me 3 months, and the last 3 weeks of that are somewhat iffy. I like a lot of the pluses of the more expensive shoes, and I will only buy the ones with the durability guarantee, so the way I see it; I can pay 50-70 bucks for shoes that are not all that great and can replace them in 3 months (at full price), or I can buy top of the line shoes for around 100-120 dollars (cages/barricades, etc) and have a top of the line replacement or voucher sent to me in 3 months.
But if you are really budget minded there are some 40-80 buck shoes that are pretty awesome and also have a warranty, thus getting a little of both benefits.
So I guess it is really a matter of if you wear out shoes quickly or not.