Here is the "flaw" in the math that a lot of people do not get their heads wraped around. It takes far less than 1 watt of A/C to remove 1 watt of heat from your home. So if the LIGHTS contribute $40 worth of electrical usage into the home, then the A/C can not spend $40 to remove it.

That is the magic of phase change systems. The HEAT is moved via phase change to a different zone (outside your house).
The same concept holds true with evaporation from your tank. A 10 watt fan can produce much more than 10 watts of cooling by assisting PHASE CHANGE. You can battle a 100 watt heater with a 10 watt fan. Not only that but the central A/C can then move that moisture out of the house and you still come out ahead of the head load (in this case 100W). Remember we are MOVING heat, not changing it.
So, back to your problem. Yes the A/C may cost you $40 a month to run, but not all $40 can be attributed to the lights on your tank. You have a chiller that IS contributing to the heat load of hte house, but it is also a phase change device and therefore not watt for watt in stride with the heat load either.
Lets take this a step further if we can. You did not list the TOTAL wattage of the old setup or the proposed setup. If you are using central A/C we can look at the lights first. BOTH setups will contribute their TOTAL wattage to the house. So if you replace 800W of MH with 650W of LED, your net savings in USAGE is 150W. Remember heat IS energy, so that is ALL you save in heat load to the home, save the chiller. (read on...)
Now here is where things get a bit more fuzzy... The LEDS do not heat the tank as directly as the MH. That means that the Central A/C will do a lot of the work in keeping the tank cool by removing the heat generated by the LEDS before it gets absorbed into the tank. Note that it [the A/C unit] still has to remove the heat! The difference is that you save the energy usage of the chiller, plus a few percent that the central A/C would have had to remove due to the chillers own heat load on the house). In other words if you KNEW how many hours a day the chiller ran, you could work backwards and pretty much determine HOW MUCH you will save.
I hope that makes sense.
As for your savings plan... Sure it looks like a rough estimate that will work. You will save some electricity every month. Bulb replacement should save you money (if the LEDs do last as long as they hope in this system). Your sale of your current equipment will offset much of the purchase price and you do not have to invest in a chiller.
We could certainly talk about some of those numbers (is a chiller realy needed or would evaporation handle hte heat load). We could talk about the money LOST in the purchase and sale of equipment (it has to be considered when you calculate "savings")
But all in all I think you should come out ahead quickly enough (it may be a bit further out than 2 years...)
Each situation is different and these are the things that need to be looked at if COST is the driving factor.
Bean