I will say this, there aren't many players who are getting paid by racket companies that endorse a separate string brand (only lower ranked Head players are allowed to use string stencils that I know of ) This is because the racket companies usually won't contractually allow it or they take out the bonuses in their contract if they do ( I know this as we dealt with this multiple times!!). We had multiple top 50 players ready to switch and stencil but couldn't get it done.
A challenger qualies player is most likely just getting free string for stenciling and may or may not have a racket sponsor. challenger main draw will have a racket contract giving them free rackets, strings, etc ( maybe a performance or ranking bonus). If they are young / high upside player / former top 10 ITF junior, then they usually have an agent and get a pretty good sized contract all based on the "what could be". I would estimate between $30-100k, but that includes rackets. Yonex is over paying everyone and I could imagine they paid hefty price tags for Shapolav and Tiafoe, much more than the sum i mentioned.
Lastly, I will leave you with this. We were in talks with a top 10 player for string only and it was at the higher end of the range I gave above.