Hmm… it’s hard to tell from the picture. Is he outside at all? It almost looks like an insect sting.

Agree that is looks like some sort of swelling in response to a prick of some sort, which does welcome the potential of abscess. You can try and clean it with some plain saline, but it won’t sanitize things.

It shouldn’t be too expensive to get a broad antibiotic to prevent/treat any infection, though you’ll still need to spend money on the exam. Ultimately, in almost all cases treatment sooner is cheaper since the condition is not as bad and easier to treat.

You can ask if there are any payment plan options. You can also see here in case there’s another vet who might be cheaper/be more flexible, which being rabbit experienced: https://wabbitwiki.com/wiki/Choosing_a_rabbit_veterinarian#Rabbit_veterinarian_listings

Yes, I would have a vet look at it just in case. Especially if the swelling hasn’t gone down. Continued swelling suggests something more is going on.