My understanding is that XSI is what you buy and 3.9 is given to you for free.
I think you have your prices confused with the Essentials and Advanced bundels. The Soft site has all the products and bundles outline for you.
However, you might be able to buy 3.9 from another person and upgrade by purchasing your maintance, which is around $1,000 or $2,500 respectively for Essentials or Advance. There is also specific upgrade prices for a given version, such that 3.7 will cost more than 3.8 and so on. However, I have found that the cost of upgrades are essentially the same as maintanance, so you sort of get that for free just for upgrading.
Yes, 3.9 is a very mature and powerful product, so you don't have to have XSI to produce good work. However, if you are buying it retail, I don't think you have a choice anymore. Try to find someone on ebay or the 4 sale forum on this site if you only want 3.9 for now.
Hope this helps.
Urp