Are there reviews for the pattern? Reviews will generally tell you if it is well written - or not. I don't mind paying for a pattern that I really like if it gives good, step by step instructions. Patterns that are too vague (meaning I have to figure the exact measurements or what I should be doing next) do not interest me at all, whether I paid for it or it was free. Would you need to buy notions (specific closures, lining, batting), and if you would, does that put the cost above how well you like the bag?
For something you really like, I don't think $13 is unreasonable. I took a tuffet class and paid more than that for the pattern, plus had to pay for the class, the foam, and a ruler! No specific plans for it, I just wanted to. Had a great time and really like the results.