Are these sentences grammatically correct?

-J'achète une robe pull grise, elle coûte soixante-dix-cinq euros.
-J'achète un pantalon bleu, il coûte quarante-huit euros.
-J'achète des jeans noir, il coûte cent-dix euros.
(for jeans, should I use des jeans or un jean)

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Be sure to avoid run-on sentences, using a semicolon (;) instead of the comma(,).

#1. I'm confused as to "une robe" (a dress) or "un pull" (a sweater.) Which is it? If it is the "robe" elle is correct. Is it 75 euros? = soixante-quinze euros.

#2. Looks good for 48 euros. Just the ;

#3. If jeans is plural, the adjective "noirs" should be as well. As for the word there is some variety:
bluejeans = spl blue jeans or pantalon coutil. You decide "a pair of jeans" or "jeans plural." I'd choose "un jean noir. 110 euros? = cent dix.

If your textbook has vocabulary in the Appendix, check there.

Mme

#1 is actually a sweater dress, but to avoid confusion, I guess I'll choose another piece of clothing.

So 75 is soixante-quinze and 79 would be soixante-dix-neuf?
And gray (grise) would have to feminine too right?

Well, sweater is "chandail" so perhaps it would be "une robe de chandail?"

Yes, it would be "grise" modifying "robe." 79 = right! soixante-dix-neuf.

For the exams, I would tell my students to select another vocabulary word if they weren't positive how to say the first choice!

Yes, the first two sentences are grammatically correct. However, there are some corrections needed in the third sentence.

The correct sentences would be:

1. J'achète une robe pull grise, elle coûte soixante-dix-cinq euros.
(I'm buying a gray sweater dress, it costs seventy-five euros.)

2. J'achète un pantalon bleu, il coûte quarante-huit euros.
(I'm buying blue pants, they cost forty-eight euros.)

For the third sentence, you should use "un jean" instead of "des jeans" since you are referring to a singular, specific pair of jeans. The corrected version will be:

3. J'achète un jean noir, il coûte cent-dix euros.
(I'm buying black jeans, they cost one hundred and ten euros.)