It really depends on the type of invitations that you are looking at. There are some really fancy ones that it is probably easier to just order, but if you want something simple and you have a printer that will print on cardstock paper (not all printers do or look good on cardstock) then it shouldn't be too difficult.
I'd say probably not. Your time is worth something too. Check out Ann's Bridal Bargains. They have whole invitation suites for 99 cents and often have 20-30% off sales. We used them and were happy with the quality.