That's a tricky situation! I can understand not wanting to pay for a HB if you're actually likely to get a freebirth. My worry would be ante/post-natal care. CAn you get that at a reduced price maybe? Maybe a differetn midwife might be willing to do it that way, if she isn't? It's worth asking anyway.
You say just post-natal, but what about prior to the birth, are you comfortable doing that alone too? I guess you can always rock up at hospital if you feel something is untoward. Actually it should be possible to hire a midwife for post-natal services alone.

I wouldnt' be keen on that OB's suggestion either!