Adam,
Some research (Google search) of "power curve airplane" will provide the answers you are looking for. Depending upon the pitch of the propeller, most of your GA fixed pitch propeller aircraft will not maintain level flight below about 1900 RPM. Roughly, whatever speed the plane is flying at that power setting, will be the speed for best economy. It's a little more complicated than that, but it'll get you close. Best climb and glide speed factor into this too.
Should you choose to fly at a lower speed than that, in level flight, you will find that considerably more power will be needed, right up to full power around stall speed. Don't do this without competent instruction. Don't do it for very long even with instruction, as it is just terrible for air cooled engines.
You are not going to change the operating cost of a 152 much by flying at differing power settings lower than 2500 RPM. Yes, if you are flying a 230 to 300 HP larger GA aircraft, you will economize noticeably at lower cruise speed, but it still has to be a "cruise speed".
Once you've looked at the power curve theory, you can go and figure out the power curve for the aircraft you fly. Just plot the points on graph paper of indicated airspeed vs RPM. get as many points as you can. you will see that the points you plot form a "bucket" shaped power curve for your aircraft. You will come to notice that for every power setting other than the "bottom of the bucket" power setting, there are two airspeeds at which the aircraft will maintain level flight. One is in front of the power curve, and the other "behind the power curve".
Once you've done it, it will all make sense...