I've been making my meringue buttercream with dried eggs whites but it's quite costly. Is there really a reason to use them instead of fresh egg whites..since they are being brought to a temp of 140 degrees?

Not that I know of..I've used both but like you found dried whites too costly so I just hunt for liquid whites and stock up.

I use the pasteurized egg whites from fresh and easy and always have. They seem to work very well.