Fairgrounds is a national track. It gets a lot of satellite and OTB action (including Vegas) a $2 million bet might take a 17:1 down to 10:1.

And, actually, my complaint is that they essentially used the plot from The Sting for this episode - with updated technology, of course.

Here is a
https://goo.gl/obs8WA
recent betting payout sheet. You'll notice that one of the horses (Baba Booey) was paying 22 to 1 on a win. You'll also note that the trifecta was 20.60 on 50 cents, so that was over 40 to 1 on that.

payouts are determined by how much money is bet on the horse, the kind of money he was betting would bring the odds down to just about even money, it would never payout 10-1 and 50-1 would be impossible,

Depends on the horse. I've seen payouts of 22 to 1 on horse races. A 10 to 1 payout is not out of line, but he was also betting on the trifecta (the first three horses, in order), so a 50 to 1 payout would be more in line. More than enough.

I was totally lost re the scheme to trick Garcia about the horse race
Very simple, they knew that he owed 20 mil but only had 2 mil and wanted to bet the 2 mil on a horse race, they fixed it so he would lose and be in deep trouble with the cartel,
as far as being important? that depends whether you want to watch a fantasy like Superwoman or if you want facts that portray the FBI and NCIS. there was no way that whole horse racing part of the episode was anywhere near possible, just totally wrong in every way,
I believe those kind of episodes are stupid, but the actors can only say what's on paper.

I don't know that it's terribly important to overall understanding/appreciation of the episode but I was totally lost re the scheme to trick Garcia about the horse race. Is it worth watching the episode again to get this straight?