Well I'd say this is targeted more towards people who have already done it. 10-25 bucks seems to be the going rate (25 dollars for a non-trivial/non-100% NLP task? Come on) now to write it from scratch is not worth it unless it's like, a half hour thing you already know how to do. Now what would be worth it is if you already had a solution that you don't mind selling. Like the zip example, supposing I'd already written something like that the five minutes to look at it, make sure it works, maybe tweak it for 10/15 minutes to suit what they want and then upload it for 25 bucks seems worth it especially since I'd guess the majority of people posting bounties are going to take the first working example.

tl;dr: post code you already wrote and then get a coffee with the proceeds.