How much more economical are planes with retractable gears vs. those with fixed gears?
Consider the Piper Turbo Arrow IV vs. the Piper Turbo Dakota... Same max gross weight, same engine, same wing, but the Arrow (with retractable gear) is nearly 12% faster at full cruise power.  This would imply that, at the Dakota's cruise speed, the Arrow should burn roughly 14% less fuel (give or take a little; it's 75% vs. 65% power, but at different altitudes).  Given this, the Arrow will cost less to fly for a given trip ($/nm).  The trade-off is that the Arrow costs more to buy, maintain, and insure, while having a smaller useful load.

Note that I chose these models for a reason: The Turbo Dakota is literally a fixed-gear version of the Turbo Arrow IV with no other major changes.  In most cases where a model is available in both fixed and retract, the retract has a more powerful engine.  This makes comparisons much more difficult.  YMMV