Why do restaurants double or triple food costs for rehearsal dinners?

I wouldn't say it's for a rhesearal party I would just say party of (how many).

Wow. I didn’t mention it but the places we were looking at either had posted menus for private parties or an obviously standard menu they sent us. I’ve heard of this issue in general, but I haven’t found anywhere that gauged for weddings (maybe other than the ones you have no choice to tell like photographers and the venue, lol).

We got married in Carmel and pretty much every restaurant we contacted had a food and beverage minimum for a private party.

Get creative. Are there non restaurant spaces that will let you rent a room and bring your own food (not DIY catering, but bring in professional catered buffet)? Breweries? Wineries? Do any of your favorite restaurants offer ‘group catering’? Look at newer restaurants who are looking to gain a following. I know this is all easier said than done, so just giving you some ideas. 😊

We didn't experience any differences between the private party menu because it was a rehearsal dinner, but Sonoma is also such an expensive area to begin with and I'd guess your September date puts you in the early part of "harvest season" for all the wineries, so they are usually really busy. Those factors are probably compounding the issues related to the quotes you're getting. It sounds like you're looking at higher-end, more specialty restaurants. If you're committed to that kind of a venue, it sounds like you'll pay a lot. If you're open to a more casual, less expensive vibe, I'd look for restaurants that are more in the "fast casual" category. Daughter & SIL's RD was at a BJ's restaurant. They charged us per person for one of their buffet options (we did an upgraded one that included an entree along with pizza, pasta, and salad). We added a full hosted bar. For about 40 guests (several of the OOT guests attended), with tax and a generous tip, I think we came in under $1500. They let us bring in a cake, and were pretty good about us being there for about 3 hours (their usual private party time is 2 hrs). There are less expensive options, if you're not set on a formal dinner.

EDT: Sorry, I don't know why I thought your date was September.... Clearly, April is NOT harvest season.... LOL!

Thank you for the comments and advice everyone!

It is true that I am planning to host a more formal dinner. And I will definitely look into newer restaurants that will just let us sit in the main dining room! And will broaden my search to more casual locations such as a gastropub. I'm hoping to keep costs at $100 per person including tax, gratuity and beverages.