Hi there - I'm Dr. Sara. I'm a licensed veterinarian who works exclusively with dogs and cats. I'm sorry to hear that your buddy hasn't been feeling well.
When I see a pet suddenly licking and chewing like crazy, especially just after a bath or trip to the groomer, my first thought is that they may be having a topical reaction to one of the products used on them. In this case, I'd suggest bathing him in cool water (this will help soothe the itching) and baby shampoo, making sure to rinse really really well.
When a pet has severe itching especially focused on the hind quarters, flea allergy is also a top concern. Some dogs are so allergic to fleas that just one bite can send them into a complete frenzy of itching and chewing. In these cases, you won't ever see the fleas. It may be that he was bitten at the groomer, the fleas did not follow him home, but he is so allergic that they made him quite itchy. It's important to make sure that you're using a good quality flea control product like Advantage/Advantix, Nexgard, Frontline, or a Seresto collar. Actually there are many flea control products on the market that work well, the above products are the ones I use most frequently.
For a dog who has allergy itching, a dose of 1mg per pound of benadryl up to every 8 hours can also be helpful. Discuss this with your vet if he's got any other health concerns or is on any other medications.
If this has become an infected "hot spot", though, he's very likely to need antibiotics and a course of steroids prescribed by your vet to calm the situation down. I wouldn't expect it to cost $1000, or anywhere close to that. In a real financial pinch, I'll often write prescriptions for my patients' medications that can be filled at a human pharmacy. Some of the most commonly used antibiotics and steroids are on the $4 list at WalMart and other grocery stores. I would still contact your vet to get an idea of what to expect financially.
Please let me know what other questions I can answer for you :)