Upvote:3
My usual rule of thumb is that one mile is worth about one cent, based on the figure that 25,000 miles buys one restricted domestic round-trip which can usually be found for around $250 on the open market. One should probably discount the miles further compared to cash, since they can only be spent on one thing, can't earn interest, and can expire (as you found).
Personally, I'd want to have almost the full 25K and a firm plan for when to use them before I'd consider paying a reinstatement fee.