“That's as low as I'm willing to go.”
You're trying to sell your motorcycle. You're negotiating with someone who might buy it. You've offered to sell it to them for $7,500, but they're asking for a lower price. You don't want to sell it for less than that amount, so you say this.
That's as low as I'm willing to go.
The speaker uses "that" to talk about a price that she previously (already) said.
If the speaker hadn't said the price yet, she might use "this":
OK, I'll give you a little discount. This is as low as I'll willing to go. I'll give it to you for seventy-five hundred, and I'll throw in the cover and helmet for free.
When you're trying to sell something, to "go low" means to offer to sell something for a low price. Here are some other examples of how to use it:
Is that as low as you can go?
I can't go any lower.
When you say that you "are willing to ___", it means that you will say "yes" to doing it, but you don't really want to do it, or you're not happy about doing it.
For example, if your lazy grown-up child asks you for money, you can say:
I'm willing to lend you the money, but I expect to be paid back by the end of this year.
Or if someone offers to buy something from you, and the price they offer is lower than you want but OK, you can say this:
Yeah, I guess I'm willing to sell it for that.