Thing is, most people see wedding invite & automatically think the words "obligated" & "fund raiser" Heck a lot of folks hold destination weddings knowing that many won't be able to come but will send a check along with their regrets.
People need to remember that at a wedding, the comfort & consideration towards your guests is most important! My wedding was cheap but within my means & put the needs of our guests first. No cash bar, no charging admission tickets or doing money dances at the reception. We had good booze, plenty of beer, wine & soda. We had the Chinese dinner & all guests invited to the after party.
I see people inviting folks to big church weddings but not inviting them to the reception or worse, holding a sit down dinner reception for some guests, followed by a cocktails & snacks reception for others. Absolutely atrocious etiquette!
You have a guest list of over 100 people? Cut costs where you can, get married off season, see if you can marry in the rectory with just intimate family & throw the money on a reception but don't cheap out on the obligation to be great hosts. Remember, this day is actually the first time you & your wife will entertaining as a married couple. Don't cut corners on the comfort of your guests! A dry wedding? Not popular but doable, just beer, wine & a champagne toast? Totally acceptable. A morning wedding with breakfast, mimosas & champagne? Great! An early afternoon wedding with substantial apps & cake served instead of a sit down dinner? A marvelous idea. You guests must be fed, given something to drink, not left to sit outside in the scorching heat, not left fighting for chairs.
Think about your duties as host & plan with your guests in mind. It might well be that mother in law needs to be politely put in her place, cut the guest list off at 100, she gets 65, you two get your 35 friends. You already sound like you aren't looking forward to one of the biggest days of your life & that isn't right. Your wife to be must talk things over with you & agree to present a united front with her mother. If MIL isn't onboard, save your money & hold the wedding you can afford. If your wife to be caves & sides with her mom, you have a picture of exactly what your marriage is going to look like.