You don't need that much rock, 120lbs is quite a bit for a 90 gallon but of course still doable.
As already mentioned you also don't need to use live rock, my first tanks were setup with almost 100% base rock which doesn't always look quite as nice but it's way cheaper, (50lbs for less than $100). Plus in my mind it's just rock and what you put over it matters more. You can however get nice looking base rock but it costs a little more but still usually cheaper than live. Dried rock should weigh less than live so even if the price per pound is the same you should get more with dried.
First reef tank (100% base rock)
