Notes on addressing invitations:

Formal invitations are usually addressed in a formal manner, that is, use of proper titles and no abbreviations or nicknames. It is helpful if your list is typed up this way (Mr. and Mrs William Smith, 2213 Upland Avenue, Big City Michigan 89098).

Titles such as "Dr." are sometimes considered business titles and not social ones, use your own judgment depending on your knowledge of the guests' preferences

If you are having a more casual event and want to throw the rules out the window, go ahead, but realize that "Mrs." is considered to indicate "wife of" so "Mr. and Mrs. George and Sally" would be incorrect, though widows are sometimes called "Mrs. Sally Smith". You may either use the titles with the husbands name or skip the titles and use first names.

If you have inner envelopes, they usually have the first names of all the invited members of a household. It is proper to have the actual name instead of "and Guest" whenever possible.

There are websites that have advice and allow you to enter and print addresses if you like.

Price estimating:

Prices vary according to many factors such as the type of paper, lined envelopes, color matching, rush jobs, etc. Most jobs range between $2 per envelope minimum up to $3.50 per set max.