Build yourself a 3 speed IGH Dahon Boardwalk new for around $250.
Thor has some older new old stock Dahon Boardwalks for $150 shipped in the USA. Utah Trikes has the 3 speed SA hub/rim with shifter for $50 plus shipping. You would just need to come up with a rear brake and you could have a decent three speed at about half the cost of a Curve.
http://www.utahtrikes.com/PRODINFO-S...Wheel_Kit.html
http://www.thorusa.com/dahon/accessories/oneoff.htm
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Speed Uno
Dawes Kingpin 2speed