I have been researching this over the past year ... There is no way you can mix, bake, cool, decorated and store enough cupcakes on a food truck to make any money. You need to have a separate location to do all that, load your truck and go sell them ... besides a nice hot cookie out of the oven is not that same as a hot cupcake, hot cookies don't need to cool that much, don't need to be decorated and the dough can be stored in a cooler and baked on location, you just can't do that with cupcake batter.
As for the truck itself, you can get something as small as a Dodge Sprinter and have it customized for around $30K, you really don't need to invest in a huge truck unless you are going to offer other types of dessert and do some baking in the truck.
I agree with Jason, unless you have a business plan, know your area you plan to market in and have a partner to help with everything, don't waste your time or money. The best place to test your product & market is at a local farmer's market or holiday markets. Good Luck and we hope to see you on the street some day