1-1.5 per gallon is what you want if you are planning on using your
LR for filtration.
If you have some other form of filtration such as a
HOB filter or a Canister Filter, than you wouldn't need so much.
More importantly, (accually just as importantly) Your fish will need enough structure in your tank to feel secure. Think about where they live in the ocean. There is tons of rock and structure there. right now you have them in a glass box with a little bump in the middle. By adding rock, you are accually helping to make them more active and act more naturally. (plus they will be less stressed and will live longer.)
If you are worried about viewing the fish, don't be. The Fish will always be easy to see. Plus, The Live Rock will add so much drama and appeal to your tank that it will amaze you. I spend more of my time looking at my rock than I do looking at my fish. There is so much to discover on good live rock.
If expense is what is holding you back, why not try getting another 10 lbs from a great online dealer like liverocks.com and then about 40 lbs of base rock. which will eventually become Live.
check out the gallery to see some members tanks. You will notice that the tanks that look the best are the ones with a good amount of rock. And chances are, those tanks have the happiest fish too!!!
-Matt