It depends on the system and the warranty chosen -- it does cost quite a bit for an extension vs. an extended warranty up front, regardless of the system.  High-end notebooks can easily run over $400-500 for just a year's extension -- desktops (assuming the T7800 is the one you're asking about) should cost less, but will likely still be a couple of hundred dollars.  There's little inside a desktop system that's very expensive - unlike a notebook, where a system board can cost upward of $1,000, so it may not make economic sense to extend a warranty on a desktop system.  
Hello,   I'm curious to know how much would cost to extend my current warranty by 1 year  , or by 2 years. I think SupportAssist or another utility shown it, a few months ago ( I may easily be mistaken ), but definitely i don't see the information now. So, I would be curious about this + if there's a time limit ( like , can it be decided and done 1 day before the warranty expires ? ) Thanks
Question about extending a warranty