For the end of season, your coaches will sign an extension as long as they continue to make the same amount they currently make; so you don't have to offer what they ask.
To hire an unemployed coach, you can offer him the minimum right now and he'll take it (unless someone else offers him more).
I haven't had the chance to see what I can do to bring this down, my thought though is that it is the AI owners responding too heavily to the market and it's getting the assistant coach offers out of whack.