the 150 is the only OTA only STB available. it lists for $599 and currently has a $100 rebate from Samsung (i don't know if you are aware of this and have included it in your $400 price.)
Samsung is supposed to release three new STB's in June, including one that will list for $399, i'm thinking when it becomes readily available it'll got for $325-ish.
you can also use any of the DBS units as OTA only, the satellite portion of the box will just be idle. but as you probably have found out, you will need to purchase these as if you are an existing customer, at a higher price than if you wanted to activate a new account.
how's your reception in the valley? i'm on East Hill in Silverton proper and get great analog reception, i'm not yet HD ready.
Have you checked your Best Buy for open box items? I got an RCA DTC 100 open box for $120 in Indiana. Thus far I only use it for OTA. I've seen open box items at other Best Buys around $300 also. Good luck!