Excel isn't a "fertilizer". It is a carbon supplement. It doesn't replace nitrogen, potassium, etc.
RootMedic is the "cheapest" liquid fertilizer option you can do. I might be biased(I own rootmedic...) but there isn't any question there. The CHEAPEST option for you is a "dry" fertilizer using a system like EI or PPS pro(now I don't make any money if you buy dry...). You can get them from greenleafaquariums, aquariumfertlizer.com, or laaquaria.com. I've had good luck with all of those stores.
If you really want liquid, you should know on a larger tank you will go through a bit more, than the 6 dollars per bottle you are use to right now with excel. I wouldn't use excel on a tank that big... After a year you will have fully paid for a pressurized co2 tank. Also, moss and excel often do not mix well.
But lets get back to THIS tank. What are your light plans for this tank? For liquid ferts, the options are: RootMedic micro/macro, pfertz NPK+M, ADA, or Seachem, which you would need N, P, K, and Then comprehensive.
If you want cheap, look into a dry dosing system. None of the liquid ferts worth a darn are super cheap.
And for rootmedic, use the coupon code "plantedtank" to get 10% off at www.rootmedic.net