I am paraphrasing some more knowledgable posters on the IMDB board for this film, but apparently the bartender's comment was aimed at Jack's attempt to pick up Jimbo the rodeo clown - the suggestion of the comment being that Jack ought to try with a man with less experience, a younger and naive person.
Jack, angry over being shut down and having the bartender notice his (attempt? failure? who knows?), determines to answer the bartender literally with the "do I look like I can afford a ropin horse?" and stomps off in his anger (and/or fear?).
Yes, I think Jack is pretty angry because of the bartender sarcastic comment, that emphasizes his incapacity of roping anyone in,
especially Ennis, of course
who he literally caught with the lasso , without succes, on their last day at brokeback
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By the way the (horrible) italian translation of the dialogue is more explicit:
Bartender: Why don't you try with the lasso?
Jack: Do I look like one who has to pray someone?l
So maybe I'm inluenced by that...