Even splitting the cost over 3 updates, its still too much money. I think $5 -$10 is fair.
As far as the 'It took 10 months since it involved coding...' statement goes, I saw how long it took to update RBL2, it wasn't 3 months. It may have been 3 months work but it was more like 12 before it hit Itch. I can go check right now if you like.
Also if you wanna prove that you can get new content releases out swiftly, do it. Prove it.
I'll eat my words if you provide a content release within 6 months, same quality and amount of content as this one, while also maintaining your work on RBL2.