I'd imaginge it very much depends on the kind of stadium you want to build - that is, what kind of infrastructure you want to incorporate (e.g. underground parking, shopping mall, luxury suites, hotels, transport connection, retractable stadium roof, etc.), and what kind of capacity you need.Aren't soccer-specific stadiums relatively cheap compared to other major sporting venues? Bah. I'm not a United fan or anything, but they are the most successful team in the league, that should count for something.
Yes and No. A soccer stadium and a football stadium are very similar in design. They are decedents of a common ancestor after all. The costs are going to very similar given a similar capacity. Soccer matches are not going to draw the 60,000 fans a NFL game would draw. That allows them to build smaller and cheaper fields of around 20,000 patrons instead. If Soccer drew the same crowd, the costs would be about the same as a modern NFL stadium.
In the end, I very much doubt there would be a significant difference in construction cost, if, say, the Allianz Arena in Munich was a pure Football stadium or Gillette Stadium in Foxborough, MA was a pure Soccer stadium (I guess a direct comparison of the two wouldn't be feasible for many reasons, although they're comparable in capacity and date of completion/opening).