Setting up pricing for a complex product

I have product rendering agency business and want to refine my pricing strategy. On initial calls with possible clients the conversation goes like this: "I need renders for my product, how much will it cost?" "Well it depends on a lot of things; how many do you need? How complex is the product? Etc."

I often stumble and have problems just throwing out a number because the price depends on a lot of factors. If I give a standard price for a certain type of render then i could be easily undercharging or over charging resulting in a lost sale. I typically target small businesses, entrepreneurs, and smaller product development agencies as they do not have an in-house cg artist. This post may not match the target demographic of this site, but the pricing concept still applies. If you’re an app development agency, how do you charge? If it’s a simple notes app with a simple ui? Or a complicated app with multiple backends and databases? How do you itemise things on the invoice? What’s the typical sales call like?