Yes, good show tack can rack you upwards of 5-8k per horse. Your show clothes (shirt, tie/rosette, pants, boots, chaps and hat) are another 2-5k. Of course, you can go cheaper if you are in 4-H, but I used to show 4-H, local and Quarter Horse shows. Tail extensions are very expensive and ridiculous in my opinion. Unless you have a horse you paid 8-15k for and it's already a push button horse (very well trained and needs no other training), yes, trainers are expensive. A cheap one is $400 a month and the price goes up as the quality of trainer goes up and the level of training goes up. If you don't know how to ride your horse or need the extra help to keep you and your horse in show condition, then you have lessons. Again, depends on the trainer, but cheap would be around $30-40 an hour.
I'm a back field friend type of person. Although, I do not really ride anymore. I want to, but I keep getting horses I can't ride. I lost my confidence after a few bad spills and haven't found that horse to help me get it back. Mom's mare would, but she's to small for me to ride. Instead, I brush her and mine and feed them treats.