。
She'll probably pay 50 dollars at the most.
Literal
She [topic-は] at-most 50 dollars only won't-pay probably.
Three layers stack here: せいぜい ('at most') as upper bound, ~しか~ない ('only', requiring a negative verb) for emphatic exclusion, and ~だろう ('probably') for epistemic uncertainty. ~しか~ない works in mirror image to English 'only' — the Japanese verb is grammatically negative ('won't pay anything beyond...') even though the meaning is positive ('will pay only...'). Pairing せいぜい with this construction doubles down on the limiting force — 'absolutely no more than 50 dollars'. ドル is a long-established loan that came via Dutch and English routes from Spanish 'dólar'.