Read the fertilizer info at the forum wiki:
Info:Fertilizer - Bananas Wiki
Fertilizers are formulated as acidic, neutral, or alkaline. Generally, for bananas you want a fertilizer that is acidic for gardening/vegetables. But this might vary depending one your soil. ... A vegetable fertilizer will have or should have the proper micro elements that bananas need. ... Knowing that mature banana plants need 1 lb of nitrogen & 1 1/2 lb of potassium per growing season (year), then one can calculate the quantity of fertilizer and/or any supplements needed per plant.
example: I use 10-10-10 available locally at $10 per 50 lb bag. For 1 lb of nitrogen, I need 10 lbs of fertilizer (1 lb/ 10%= 10 lb) per plant. The same 10 lbs will furnish 1 lb of potassium. Thus, a potassium supplement is needed. Using potassium sulfate (0-0-50) about 1 lb is needed (.5lb / 50%= 1 lb). ...
Knowing how to calculate the quantity of fertilizer needed, you can then use off-the shelf products and compare cost.