If you are interested in MegaTraveller or The New Era, you can buy ALL of the books for these in PDF format on CDs from Marc Miller's Far Future Enterprises for quite reasonable prices: generally $35 per CD, and each has a LOT of stuff on it. If you want 3 or more, his 443 deal gives you 4 CDs for the price of 3.
Buying a bit at a time when I had some "extra" money, I have now duplicated almost all of the Traveller books that I already owned, plus gained LOTS of new material that I never had before. The big advantage for me is that I can copy the PDFs from the CDs to my laptop HD and have all of those books right at my fingertips.
All that is lacking is my World Builder's Handbook (a DGP book), some Keith Brothers' books (which are available on CD, I just didn't buy them because I don't refer to the hard-copies that often), and all of my GURPS Traveller books (which FFE does not sell, but are available from SJG, although at much higher prices per individual PDF).
There are also some reasonably priced newer setting materials (written for MgT, but you could adapt) available by downloadable PDF from DriveThruRPG.