Agreed. I believe any hobby, especially equine related hobbies, eventually become expensive. But it is well worth it. I do hunters and I love it.
As far as tack goes, you get what you pay for. If you buy cheap tack it will be poorly made and will most likely fall apart. My advice would be to save up and ask around the barn where you'll take lessons for advice and recommendations on brands, etc. For now, your trainer might have some tack you can borrow. I would look into a good set of open front boots for your horse to protect his legs once you actually start jumping. You might have some time though because my guess is you won't be jumping for a while until you have some of the basics down.
As for supplements, I would consult with your vet and they should be able to tell you if and what your horse might need.
For your apparel, unless the barn you decide to ride with requires you to ride in breeches, polos and tall boots, for now you should invest in some black paddock boots and half chaps. My paddock boots were only $35 and I love them and my half chaps were around $80. Some schooling gloves could be good too. Of course, a helmet is something you will need as well. (I'm not sure if you wore one when barrel racing, most people I know do not).
Well good luck. You'll love it. It is very addicting.