Facebook and Classmates are really good resources, though Classmates requires a paid membership to do anything really meaningful, and that's going to knock out many classmates of yours. I did NOT want to pay for the membership, so if they had solely relied on it for our 20th I would have been out of luck.
Price depends on your area. Our 10 year was too expensive for me to have dinner there, but they had a more reasonable "get in the door" price for after dinner. Alas, they'd had the "awards" during dinner! That was STUPID, do NOT do that. There were NO drink tickets, it was a cash bar.
The 20 year was even more expensive, nearly $100. Two drink tickets and a HORRID buffet. Well, for me, at least. I'm vegetarian and they had ONE item that I could eat. Bruschetta. That's it. And the rest of the buffet was just offensive to me, with legs of ocean-spiders (crabs) draped over bowls and whatnot, gag. Try to not offend or disgust people, LOL. Or if you must include those things, also include some other items as well, that will make people who do not eat such things happier.
The nice part of the two were that they had blocks of rooms in the hotel that each was held in, and the prices were very reasonable. That was nice.
They had an informal thing at the bar the night before I got there, the formal thing, and the next day they had a small spot at a local park festival. We did not go, as I haven't lived in CA for years and am NOT used to (late) summer festival weather. Had no interest in passing out from 100 degree heat. I don't remember anything beyond the dinner/reception for the 10 year.
For the 20 year, it was too expensive for DH to come. Plus, we didn't know anyone in town who wasn't at the reunion, so we had no babysitter. Therefore, DH and DS hung out in the hotel room, watching Ratatouille, while I was at the reunion downstairs.
DH's 10 year (I wasn't there for) was very reasonable in cost, invited families, and he really enjoyed himself. His 20 year will likely be reasonable, will probably involve families, and we will probably have more fun. His is in WA. So the prices are different than my Silicon Valley/Bay Area reunions.
My 20 year was in Sept, and I found that to be an odd time of year. The 10 year was more June'ish. DH's is in June, and I think his 10 year was as well.
There WILL be some families there, and so I think having it before most schools go back into session is a good idea. I don't think I'd do Oct...that seems like it would shoot the reunion in the foot, so to speak, dooming it to failure just b/c the kids that already exist might already be back to school and parents won't be able to come.
Oh, and don't focus totally on those who are still local. There will be some who have left, but might still want to come back. If you really focus on the locals, you will alienate those who have traveled, and that HURTS. If you have a "who traveled the furthest to come to the reunion" award, try to not make the winner a popular girl who came from the southern part of your state. If there are people coming from WA and GA, all the way back to CA, try to mention THEM instead. It's really only fair. Even if it's not someone you knew in HS.