If you have a big space you need to heat and purge air, read further. This isn't a residential unit, but it is single phase power.
We are cleaning out the shop here and we have a Trane 100,000 Btu/h natural gas fired indirect make up air unit. It is brand new, still on the pallet, we just didn't use it for a project and it was already rolled into the job. Anyway, it is probably 5-6 years old, we paid around $6k for the thing. This will do 1100 cfm. We have it listed right now for $1800, I can probably get it to you for $1500. I am pretty sure it is indirect fired so you could set it up as a recirc unit if you don't need to run it as 100% make up. The unit is 80% efficient on the gas side.

Let me know if you have interest. I am sure we have a submittal for it.

Obviously due to age, there is no longer a warranty on it. But like I said, if you have a warehouse/workshop/big garage you need to heat and don't want to spend huge $$, this could work for you.