[Kadelake]

I'm not sure but I think it's impossible to have a negative modifier. This is based on the bug(?) with the gem-discount sites. If you cast a global with 20% discount, let's say brining the base cost down to 80 from 100 gems, you will only receive a bonus modifier for gems above 100. However you won't get any negative modifier from casting it with only 80 gems.

But maybe bonus from spell level is handled differently.