Now you come to her motives for what she was doing here. She saw that Judah was tricking her, or keeping her from what was rightfully hers. Whether she was thinking that she would be barren (because, remember, she was probably by this time in her early twenties and beginning to feel like she was an old maid) and that Judah had a responsibility to her and he was not fulfilling it. Like Sarah and Rachel, and others, she was going to somehow take matters into her own hands and work this out. So she decides to pose as a harlot.
Evidently, this "young goat from the flock" was considered a delicacy. So it was a fairly high price.
That is, "something as an earnest, so that I'll know that you will actually give it to me."
