Learn English – Comma before “as a token of the gratitude?”

Either is ok. Commas indicate a pause. If you want to emphasize the contrast between the two clauses more, add the comma.

By the way you don't need the "to me" in that sentence; it's implied by the fact that the sentence is written in the first person ("I" is the subject).

I like your car, but it seems too expensive for you.

Advice from the BBC:

A comma (,) generally indicates pauses in speech. But, when it joins two clauses, it indicates a contrast between two ideas. In speech it is normal to draw attention to this contrast by a slight pause. A comma is the usual way of indicating this, although it is not obligatory:

Sheila can eat anything and large quantities of it, but she never puts on weight.

I'm going to make some New Year resolutions, but I don't suppose I'll keep them

Punctuation is a matter of style and preference and it largely depends on which style book you or your editor use. However, using a comma is not as much confusing as other punctuation marks.

According to the linked Commas: Quick Rules by Purdue University:

Use commas after introductory a) clauses, b) phrases, or c) words that come before the main clause... Common introductory words that should be followed by a comma include yes, however, well.

1. The word yes is an interjection and it is usually placed at the beginning of a sentence. It could be considered as a word that come before the main clause which requires a comma.

Use a comma near the end of a sentence to separate contrasted coordinate elements or to indicate a distinct pause or shift.

2. isn't he in the sentence is called a tag question which could be considered as one of contrasted coordinate elements. Therefore, a comma is required before the tag question.