They quoted me 28k for 2 heads for a space about 600 sq ft. I thought it was a load of bullshit.

Considering the only real costly part of the install is getting the electric right I would look at doing a self install if you are handy. It's mostly boring a hole and hanging the unit then running the duct to the compressor. Hire an electricians to hook it up to your box and save yourself 38k.

I also was considering replacing a little boiler instead of the mini splits and they said that would cost about 18k. For reference they replaced a huge boiler in my house for 12k and told me that a unit half the size would cost 6k more. They also told me they couldn't install a high efficiency boiler in the space because it was right but another company said they would be able to fit it.

IMO they are taking advantage of expected inflation and seriously gouging.