There are many ways to deal with this - you could ask them to do a buyout for perpetuity, which would give them exclusive rights to the image forever- for that you would want the most money. Or you could license it to them for a period of, say, two years, and ask for less, then renegotiate after that time has past. A non-exclusive license would let you sell it elsewhere, such as on Adobe Stock or Shutterstock (not a lot of money per sale, but potentially a lot of sales). You should submit all your best shots to sites like that and see if they accept them...
But whatever you do, don't do it for "the experience." They would just be ripping you off at that point.