I have tons of mobile substrate stuff installed on my 3GS and I usually have about 150-155 without anything running in the background. With no mobilesubstrate i'd have about 170.
Actually I deleted the program I had that checked free memory so I can't check again, and I only looked once... sorry. In any event, running out of RAM doesn't seem to be much of an issue like it was before, thankfully. 🙂