The first thing to remember is don't get yourself in a panic about inviting everybody you have ever met or even everybody whose wedding you have ever been to. People do understand that weddings cost money.
Easy ways to reduce your potential wedding guest list are;
~ to only invite immediate family and close friends to the reception and then invite everybody else to an evening reception. This way you can keep your costs down and still see everyone.
~ when addressing single friends don't write "and guest" on the cheap wedding invite. You know your friends and family well enough to know if they have a partner, don't feel you have to invite their latest, remember it is your wedding and you should at least know everyone there!
~ only invite adults, then you won't have to pay for a meal for the children or possible entertainment. Sounds harsh and for some people having children at their wedding makes the day complete so again it is personal preference.
~ only send one cheap wedding invite per household, they don't all need their own invite and this will save you on printing costs.
Just kidding, although I know that at some stages in the planning process you think this is probably the way to go, and it would save a fortune. Just think about the possible consequences of this action before you jump into it!
If you feel that by not inviting people to your wedding you could lose friends, cause arguments or general chaos then it might be worth thinking about some water tight reasons before you send out your cheap wedding invite.
Here are some that I know have been used; the venue is only able to hold x number of people, or we are only able to have a late ceremony and didn't want you to upset your children's bedtime routine, or we know weddings can be boring for children or the honest answer of money.
Whatever reason you both decide to use make sure you all use the same answer and that includes the mother in laws! Again remember it is your big day, the day you have always dreamed of, just make sure the people you want there are there.