Many small planes have fixed gears, some have retractable gears. All things being equal, the plane with retractable gears would have to be more expensive than fixed gears, as the mechanisms involved in retracting them is obviously more complicated than fixed gears. On the other hand, planes with fixed gears should have somewhat more drag than a similar plane with retractable gears when they're retracted. This would imply that a plane with retractable gears would consume less fuel than one with fixed gears. If the difference could be quantified, how much more economical would the plane with retractable gears be vs. one 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
One thing to consider is your drag profile and mission requirements. If you have a cumbersome aircraft with high amounts of drag cruising at slower speeds vs. an aircraft designed for impressive maneuverability and high speed, the impact of adding a fixed gear becomes much more drastic. For the cumbersome craft, going with a fixed gear might be much more economical because the complexity and cost is so much less and the added drag of having a fixed gear might not noticeably affect the design parameters. For the other example, the added drag of having a fixed gear would substantially diminish the craft's capabilities. I don't think you can simply put a standard "quantification" to the economical benefit because in the end, the benefit/detriment of having a fixed gear vs. retractable gear really depends on the aircraft.