There are certainly cheaper pumps to run, but you will likely suffer a trade off, but maybe not. A 3/4 HP sump pump will put out alot of water flow but very little pressure. You may be relying on the higher horsepower of your sump pump to produce the pressure that your nozzles require to look nice. If this is the case, your overall flow may be low.
What pressure is your pump putting out?
Can you easily determine how much water is coming out of the fountain per hour?
Do you have a pump curve for your sump pump?
Do you have the running watts for your sump pump?
You need three of the above questions answered in order to evaluate the system on paper. Otherwise, it will be trial and error.
My guess is that your pump is working harder than it needs be to create the pressure to form the streams of fountain water that you like. Sump pumps lose a lot of flow at elevated pressures (yet low pressure compared to other pumps). If this is the case, you need to know how much flow makes your fountain the way you like it and then look for a lower wattage pump with that kind of flow. This new pump would need to be a more positive displacement pump in order to supply the fountain with enough water and be able to maintain the pressure requirements of the nozzles.
I feel like I'm getting too wordy so I'll stop here and wait for the questions above to be answered.
