if i were u id jump to nikon. i feel like they tend to be cheaper when it comes to 2nd hand stuff and 3rd party lenses. but its personal preference in the end
2nd hand Canon and Nikon lenses are more expensive than 2nd hand Sony Alpha mount lenses generally, especially if u take into consideration that 2nd hand Minolta lenses are very very cheap. Btw Minolta is NOT a 3rd party brand. Minolta = Sony's DSLR division (long story, u can google for more info). Infact Minolta invented the 1st autofocus lens and camera body.
Take for example the Minolta 70-210mm f4, it cost $300 only 2nd hand. Which other brand gives u a 70-210mm constant f4 autofocus lens for this price? Or Minolta 50mm f1.4 for $300 as well (Canon ones cost around $500)
Unless u r talking about Sony's Carl Zeiss lenses, than thats a different issue. As mentioned, Carl Zeiss is a PREMIUM brand, and that is why they can command a high price due to their quality. If you compare only Sony lenses, then they're equal or lesser than similar Canon/Nikon lenses.
the only lenses that I know of that Sony price higher than Canon and Minolta is the 50mm f1.8. But then again, Sony's 50mm is stabalised whereas Canon and Nikon ones arent.
You also need to consider the fact that Nikon and Canon lenses comes with image stabalised and non image stabalised versions. On the other hand, all alpha lenses mounted onto a Sony DSLR is image stabalised. As such when comparing lenses, U should compare Sony lenses against Nikon and Canon's stabalised version of the lenses, not the unstabalised version. And in this comparison, u will know that most Sony Alpha mount lenses are cheaper than Nikon and Canon's stabalised lenses of similar grade.
That brings about to 3rd party lenses.
Tamron 17-50mm f2.8, one of the most popular 3rd party lenses used by Sony, Canon and Nikon DSLR owners.
Nikon and Canon owners need to pay $900 for the Tamron 17-50 VC (stabalised), whereas Sony users need to pay $600 only cos we dont need the lens VC, as the non VC version becomes stabalised when mounted onto Sony Alpha.
Add to the fact that Canon and Nikon dont hv much stabalised primes, whereas all Sony primes are stabalised, that is already a bonus advantage that u get without paying a cent more.