Money can be a difficult subject and asking people for money can be even harder but it's very important that everyone is on the same page right off the bat when wedding planning. Getting a clear budget is very important right off the bat for a few reasons. First off, the most expensive parts of your wedding may be the first things you do. For Eric and I, the most expensive parts were the venue, catering, and my dress. We had to pick our venue out a year ahead of time and the dress was set pretty close afterwards. Also, if you have logical family (which may be a huge if) and they hear that your total budget is $200 they won't push you to invite every single person they've ever talked to. Obviously this is an extreme case, but I'm certain that at least a few wedding budgets have been blown out of proportion by insane guest lists, and not only over the top decorations or wardrobes.
When you're meeting up to talk about the wedding budget, it's fine to go out to dinner, but be clear that you want to talk about the budget. Money is a sensitive subject and to blindside someone is not ok. Also if something is vague ask some follow up questions. Don't push anything though.
If there's something that is a want for either set of parents and you just can't afford it be direct in explaining that. For example if your parents want you to have a certain dinner for the reception and you can't afford it, be clear in that. They may have ideas to work around it. I know a person who had the ceremony, dinner, and reception as three separate events where they had close friends and family at the ceremony, extended family and friends at the dinner and then everyone from their church at the reception.
Give Them Time
Your parents or future in laws may not know how much they can contribute right at the moment. Give them some time. It may take a few weeks to figure out their home budget and get back to you on what they can contribute. They may need some time to actually contribute as well. Most things you have to pay a deposit and then the balance is due somewhere between a month and 2 weeks before the wedding so if you parents/in-laws have to contribute on a monthly or some other form of payment basis that won't derail anything in your plans.
Show you Have Some Skin in the Game
Let your parents and in-laws know that you are contributing as well to the wedding budget. I would also suggest that you are in charge of the budget instead of one set of parents saying "I'll pay x y and z" as this will avoid some bridezilla-esque fights (watch "Say yes to the dress" if you think that paying for a certain category won't cause overspending) and it's a great practice for doing a household budget for the new couple.
Take What They Give You
This should be self-explanatory but you get what you get and you don't throw a fit. On the other hand if you have to open 7 credit cards or take out a second mortgage to invite everyone on your parent's lists then there needs to be a conversation about timeline or budget.
Don't Count on Money You Don't Have
Here's the thing I wish I had known/thought of before our wedding. Don't count on money you don't have. Even if someone is offering to pay for something don't order something you can't pay for out of your pocket or what is in your wedding budget. We blew our budget sky high because we just didn't have in our account or as a check in the mail. Obviously my opinion is to not use credit cards or any form of debt, but check your own comfort level and don't go further in debt than that.