I bought our 5.5 ft artificial green tree in 2002, so this will be its'. 7th year, and the cat has climbed it for 3 of those years! It was £59.99 and still looks good. I won't replace it until it is no longer good looking or repairable. I think the better quality, the longer they keep going for. Ours is only on display for about 3 weeks, and we have no young children or dogs to knock it over, which will help its survival!
If you want a new tree, you can sell your old one second-hand in the local paper, as I did if it's still in good condition. The guy who bought ours wanted a second tree for their dining room.
My mum got mine from a charity shop for £3! I don't think it had ever been used. I've had it about 6 or 7 years and it still looks good.
Also, my mum had the same artificial tree for years and years when I was living at home. It must have been around for at least 25 years before she dumpediton gave it to my sister, who promptly threw it out because it was knackered!!!