If Stargate cost $70,000,000 in 1994, it would have cost $84,973,009.45 in 2002 (end of season 5) or $86,909,581.64 (end of season 6), using this inflation calculator (which only goes up to values of ten million, so do a little math).
They may have already had the sets and actors, but they wouldn't have come free. Surely the cast would have earned more for a big screen feature film (look at how large the cast of Star Trek: The Next Generation saw their paychecks increase for their motion pictures) than they earned for the television series. The sets would have been redressed for a larger screen, just as the sets for the Enterprise-D were for Star Trek: Generations. Visual effects would have to stand muster on 35mm film prints (more money).
I don't see a big screen version being made for less than $50,000,000 in 2002 or 2003, when the film was most likely. People weren't in the same position to want as make it (and thus, make it for less) as the cast and crew of Serenity were.
The thing is that we're jsut talking about a longer version of The Lost City and from they said was cut out I can't see it costing $50 million dollars. Martin Wood even filmed it movie style with two cameras in some scenes and the biggest cut scenes was the destruction of an aircraft carrier and it's task force and the President revealing the SGC to the public.