DoubleCAD XT -- our AutoCAD LT work-alike -- is free for personal and commercial use. We think that's a pretty good upgrade price. : )
Also, if you want more power (like the geometric constraints included in the $4,000 version of AutoCAD 2010), we're honoring AutoCAD LT's upgrade price of $575 for DoubleCAD XT PRO for the next two months -- and we're honoring this for *any* version of AutoCAD LT (not just three versions back).
P.S. Here is a link to the press release we sent out last week on this: finance.yahoo.com/news/IMSIDesign-Matches-AutoCAD-LT-iw-14733364.html
In addition, I'd note that Inventor LT is no longer free.