I had a sale on dreamstime for for exclusive rights for one year for an image and they licensed it for $750, I got $350 - it was probably at least 6-7 years ago and I don't recall how the price was set - I might have just chosen an amount at random when I uploaded the file not really expecting to get an exclusive sale, but buyers still expect to pay a lot more for exclusive use so think about what it is worth to you and give them a number & see if they get back to you. If it's a permanent buyout, it should be a substantial amount. I haven't uploaded much to dreamstime in years, and nothing exclusively anymore, but I thought when you uploaded exclusively you set the price at the time?
Anyway, good luck. It could be worthwhile.