I think the cleanest way to do it is just cap the season. We do 60 per year which is enough flexibility that if you're tweaking your team, you have enough moves, but you don't have so many that you can churn through guys nightly (it's about 2 per week).
The way we track it in Sportsline is under League Fees, I made it $1 for every pickup. We don't collect the money, just do a flat entry fee, but it's an easy way to keep track that everyone can see and keep up to date on. Also, we don't start charging until the season officially starts, so if people want to do post-draft roster tweaking if they forgot about someone or overdrafted a position or whatever, that doesn't count to the limit.
If you really want to do it weekly, you could do it the same way; you'd just have to go in and manually adjust the costs at the end of each week. More pain than it's worth IMO.
0-3 to 4-3. Worst choke in the history of baseball. Enough said.