"This shirt doesn't cost much."
Translation:Esta camisa no cuesta mucho.
Here mucho is not an adjective modifying camisa. It's an adverb limiting the verb (cost), so it doesn't change its -o ending.
I would like to know how an object can be feminine or masculine. This makes learning Spanish very difficult.
It's not difficult it's different.....you have to adjust your mind set and it just gets eaiser !