The cheapest W7 ultimate would be the OEM version which runs about US$180. Do a search for "Windows 7 ultimate OEM" to find places that sell it. Unless you need the multiple language option or the encrypted drive feature there is very little reason to get Ultimate.
Also, there is no direct upgrade path from Vista HP to W7 Ultimate so you will need to back up your data to an external drive, then do a fresh install of W7 Ultimate, reinstall all your programs, then restore your data.