The U.S. Navy has completed a six year effort to buy and modify 44 F-5E fighters from Switzerland. The U.S. uses F-5s, a 12 ton fighter roughly similar to the MiG-21, for training American pilots to realistically deal with potential enemies. The F-5 is normally armed with two 20mm cannon, and three tons of missiles and bombs. The U.S. Navy modified and refurbished the Swiss F-5s so their performance better matched that of Russian or Chinese aircraft.
The Swiss once had a fleet of 110 F-5s, but have been selling them off for the past decade. They have 56 left, and a recent opinion poll showed that 65 percent of the voters want to keep them (possibly because the government wants to buy $50 million aircraft to replace them.) The U.S. paid the Swiss about $1.2 million for each of the F-5s, then spent a little more per aircraft to refurb them.
The MiG-21 is still widely used. It is a 9.5 ton, 1950s design, the most widely produced post World War II fighter (over 10,000 built). It is cheap, and easy to maintain, but not so effective in the air. Many nations keep them in service because of the low cost, and because a wide range of avionics and weapons upgrades are available. Not really designed for ground attack, but can carry 1.5 tons of bombs. U.S. pilots are much better at killing MiG-21s once they have trained against an F-5 being flown like a MiG-21.