Adjust high nTransactionFee warning to reflect market prices.

The 0.25BTC -paytxfee warning has been there since at least commit 7d80d2e3d7ecceb5a99fef357b118949f6dd8a79 in 2012. In line with new market prices, I've adjusted it to 0.01BTC or $4.75USD at time of writing. While this is low, I think that setting it low will ensure we don't have to change it for a while. If Bitcoin goes up to $1000USD again then 0.01BTC will be $10USD, which I think is high enough to deserve a warning.