Does anybody make an inverter air to water heat pump that is good down to 5 degF and is available in the US?

The altherma does and I just ordered one for a project. They are still available.

What's a ball park installation cost for unit to tie into an existing radiant system heating system on a house with a 40k btu/hr load?

Without knowing the peak water temp requirements and output capacity requirements (at what outdoor temp) there's no way to even ballpark it, or whether if it can even can be made to work with your existing radiation.

I researched one for a client and found I could do a WTW geo with loop system for less (after tax credits). The equipment actually cost more for the air source but without the loop field cost less to install.

As Dana has pointed out there a wild deviations in installed geo prices so what is true here may not be where you are.