USDF runs the L Program as the first step for learning to be a judge.
There are requirements regarding earned scores.
You have to wait for some GMO to sponsor an offering and the price depends on what that group charges.
IIRC when I did part 1 it was around $1,000 dollars. That’s just the fee for the class. My group was held just far enough away that I stayed at a hotel for the 3 sessions so add on that and food for 3 weekends. I was close enough that I only needed 1 night at a hotel. Some flew in the day before so airfare and 2 nights at the hotel for them.
Part 2 was a year or so later, was more than $1000 IIRC. On the up side, it was closer to my house so I only did the hotel thing for 2 of the sessions.
There is also a reading list, so you can spend even more money on books. The single best one on the list, IMO, is the FEI Dressage Judge Handbook.
That all said, it is probably the single best thing I did for my riding and overall dressage education. I’m too old to make it through the r and R programs in a reasonable amount of time and I’m not inclined to leave So Cal for a program although I know people that have. It’s been around 5 years since I graduated, there hasn’t been an r program in California that I’m aware since then.
You can be a judge and retain your amateur status for showing.
ETA: Another cost is for a scribe for part 2. I was lucky and had friends that didn’t charge me, but I did pay their expenses for the weekend and buy them gifts.