I recently went through a very similar ordeal. After kicking ass for two out of three months of contract-to-hire gig, the company said they loved my work and wanted to end the contract early. Awesome. Then they dropped a full time offer for 20% less than the minimum they advertised on Angelist, well below the market. When I tried to negotiate towards something more reasonable, they pulled that same line about the range being for a very senior dev and were resistant to negotiation, even though the advertised range is big enough to fly a jetliner through and they clearly know that no self-respecting senior dev would work for the bottom half of the advertised range. They just closed a huge round and could have paid more, but chose not to.

I ultimately declined their final offer which they seemed pretty surprised by. It's hard not to take personally after working there and proving my value for two months. Instead of a bum offer from a company I just met, which is no big deal, it really felt like people I thought were my friends were trying to take advantage of the situation. I probably will never do another contract-to-hire. I'm not interested in companies with a try-before-they-buy feeling about candidates, because I think it belies an unwillingness to value and respect the work of talented hard-working technical people.

I made some money and got valuable life experience from it, but I'm still sad about how it went down. There's enough credible salary information out there that companies are really doing everyone a disservice by pulling an extreme lowball on a candidate they want.