I'm afraid that OnLive games will cost just as much as their retail-box cousins ($40-$60).
PC games are generally $10-$20 cheaper than their console equivalents, but I don't expect OnLive to follow this discounted pricing model that the PC has established.
While it's entirely true that OnLive could sell their games significantly cheaper than current retail prices, OnLive has nearly no incentive to do so. Why would they charge less than what consumers are already willing to pay?
If anything, Steam's digital distribution service proves that not only are consumers willing to spend just as much on a digital copy of game media, but they're actually willing to pay a so-called "Steam-tax" for the convenience of digital distribution. The "Steam-tax" refers not to artificially inflated prices, but to an aversion for the expected price-drops of older titles.
If you have any concrete information (or logical speculation) about the pricing model OnLive will use for games, share it below!