Well, it's current setting is ineffective. If you put money in overnight, you get 3% interest on it, and only lose 1.5% when you take it out. It doesn't really lock your money in. Plus, you can take it out in small quantities so you don't suffer the penalty.
Same thing if you increase it. It'll just make people take ore out in small quantities so they don't lose so much. All that will really do is annoying the people having to make their withdrawls in small quantities.
I guess since it's rather ineffective, it doesn't matter too much what happens with it. Although, it seems like it's a bit of an annoyance. Might as well just get rid of it.
Btw, it withdraw 500 ore, the fee is only 8 ore. Hardly significant.
Maybe if you limited the number of transactions that could be made in a month? Add service charges, put chains on the pens, etc. You might actually start earning a profit and attract some business. J/k
But seriously, you could remember the rounding errors, so people can't cheat the withdrawl fee. Then there would be more point to it. You could also try adding a charge if they try to make more than a certain number of transactions per month. That would help keep money in it's place.