... Do we get modding upgrades after so many achievements, ala Team Fortress 2?

For the PC everything is/will be free, just like the TF2 updates. The 360 is different because Microsoft wants them to charge for their DLC, which is why they've waited a bloody long time to release it.

Gabe interview: http://www.1up.com/do/feature?cId=3170080

PS If you haven't already, go to interlopers.net with your map. Hell of a community that will give you all grades of feedback.