Can the Sox afford to overpay for Crawford? Absolutely.
His value as a player: Very good defense, skillset that should age well, bases stolen success chance is not the total indicator of his value on the basepaths (i.e., if you steal 100% of bases attempted, but only steal one base, you don't give much value; if you steal 80% of bases attempted, but attempted 100 times, your value increases a lot), decent amount of power
His value in the market: Inflated this year, there's nothing that the Sox can do about an increase in spending overall. Furthermore, there are going to be few good OFs avaliable in the next few years. The Sox have a hole in the OF with Drew and Crawford leaving next year and unlikely to be resigned, and Ellsbury potentially leaving a few years down the road. Carlos Beltran, Jose Bautista, Corey Hart, and Ryan Ludwick are among the best options avaliable next year (not including Drew). The only two impact players avaliable next year are Fielder, Pujols, and Reyes (assuming Uggla resigns and Cano's option is picked up, and the fact that the Sox probably aren't interested in them); Pujols won't be paid to just DH, Fielder is also likely to be overpaid to just DH, and Reyes has a lot more question marks. I believe that the market for OFs will also be pretty weak two years after this one, but I can't find a list so this would be just a hunch. There is no better time for the Sox to add an OF than now, and it ensures the team is set and doesn't have to risk giving Reddick another try at an MLB job in a year (fifth time lucky?).