As much as I would love to have a friends movie I dont think it would be the best idea. They ended with a great ending and ended on top they shouldn't' tempt fate by messing with that. If they made one I would be there no question, and I would be super excited, but that still doesnt make it a good idea.
I think the biggest problem with the movie is transferring a sitcom atmosphere to a movie. Sex and the City and the Simpsons dont have laugh tracks for one which makes them easier to transfer to the big screen not to mention watching shows on HBO is almost like watching a movie anyway. Also the camera angles and sets and all that would be different now that its a movie which would completely change the atmosphere of the show. That kind of stuff might not effect the show in a negative way but if it did it would really ruin the movie not to mention the great ending to the series.
As for money well I think since it was one of the most successful shows of all time I think it would be a box office hit no matter what unless the movie was just flat out that bad which I know it wouldnt because the writers wouldnt make a movie unless they really thought it would be good not to mention the actors wouldn't sign on unless the script was good.
Which finally leads to getting the actors signed to do a movie. For the most part I think all the actors are busy maybe not in the super celebrity spotlight, but still doing other things which is what I think most of them wanted to do anyway especially Jennifer, David, and Courtney. So in the end I think if the studio, David, Marta, and Kevin became serious about the project and wrote up a good script the actors would do it. Having said all that I think the writers have made there piece with the show being over and just plan on leaving everything as is, and you can't go wrong with that because the ending was great.