Conversion is primarily based on value of the product to the user and the price. If its valuable and free, you can expect maximum conversion. If it is arguably valuable and not free, the conversion rates will generally go down as the price goes up.

Suggesting there is any reasonable expectation in general is really not at all possible. Find comps for products that are the same or similar, check out their prices and find out their conversion rates. Only then can you start to formulate a reasonable expectation for your specific product.