So I have a problem with excessive wear/holes in the crotch area of jeans, pants, etc. From what I can find online, this seems to be a common complaint. The obvious solution is to lose weight to reduce the amount of "friction" (and yes, I'm working on it, but in the meantime I need to have something to wear).
I weigh 120 lbs and still have this problem. It's not (just) a weight thing - a lot of women have to deal with this. Outside the sewing advice, I suggest trying different materials. Pants made from technical fabrics (e.g. nylon) are much more resistant to abrasion. Anecdotally, heavy cotton fabrics don't hold up nearly as well as nylon pants in the thigh areas. Synthetic fabrics do have other downsides (breathability, odor-resistance) so you'll need to see what works for you. These days, you can get causal and professional styles of pants (i.e. not too nerdy) in all sorts of fabrics.
Aside from that, the thrift store is always a good option. If you're going to wear through pants quickly, you might as well pay as little as possible for them. I have one pretty expensive pair of pants in a technical fabric, and I realized that even if they last me 10 years, it will probably still end up being cheaper to just by jeans at the thrift store.
Edit: also, if they fit, I believe Tilley has a pair of lifetime guarantee pants in nylon. Their smallest size didn't fit me - so disappointing!