“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:

That's as low as I'm willing to go.

that

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.

go low

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.

(someone) is willing to (do something)

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. In the example above, the woman wants to get more money for her motorcycle, but she will agree to sell it for that price.

Another good example is a teacher saying:

I'm willing to give you an extra day to turn it in, if you promise me that you won't tell any of the other students.

Read this set of questions and answers for more discussion on how "willing to ___" is used: Could "be willing to___" have another meaning?