Solar Power for a pond pump.

Hello Michael.
First of all, there is nothing inexpensive about renewable energies. Its a sellers market.
I will try not to get overly technical but without being so simple as to be insulting. Thats not easy for me. I need space to move around in.

What you really need for the system you described is a PV direct pump controller or Liner Current Booster (LCB). It can make the adjustments in volts and amps from the PV module necessary to operate a pump motor that needs a voltage and amperages different to the PV module. Also, look at the wattage difference in what you have. 
2.2 X 18 = 39.6 watts PV source.
versus
4 X 13.6 = 54.4 watts motor load.
If you want to go PV direct then you will need more wattage to use an LCB.
http://www.solarseller.com/dc_photoswitch__dc_timer__voltage_controlled_switch_dc_relay.htm

If you use batteries you will need to know how long the pump is to run at night. I will use 6 hours for an example.
Using the wattage of your pump motor 54.4 simply multiply by 6 and you have 326.4 watthours. Then divided that by the nominal voltage of 12 and we have 27.2 amphours. But! You would want to "bump" that up by a factor or two because if you didn't it would kill the battery dead in no time and you would be replacing batteries quite often. In your case I would say two - 6 volt - 220 amphour deep cycle batteries wired in series would do nicely and they are a commonly found golf cart battery. Of course a simple charge controller rated for the 40 watt PV module you have would be necessary and an enclosure box. Also, depending on just how often it was used at night, it may need more PV or a supplemental charge from the grid.
http://www.solarseller.com/solar_charge_controllers_from_morningstar__powermax_and_others_.htm

As always with electrical its important to look at and match the rated specifications of all devices used in a system. This is, I bet, what can be the most fatiguing. Imagine if this was the major part of your daily job chore. Sizing everything electrical per NEC, National Electrical Code. It does get easier after a time though.

As for keeping cost down...you just have decide what you want from the pump and do some comparison shopping. If you find that you have to replace the pump/motor anyway, look for complete kits where everything is already sized for you. I used those links above only as a comparison. Our host, Alternative Energy Store, should have these same devices.