It sounds like you are dealing with a product/application which you may have custom installs of. In this case, I would probably bail on the form and divide concept. Some assumptions based on the description:
- 100% of users who buy will pay at minimum the base price
- 60% of users will only pay the base price, because that's all they really need
- 40% of users will find the product lacking in some functionality or the other specific to their situation and will, therefore, pay more for you to customize and maintain it
The form with 20 fields is to try and distinguish between these two groups. If you can relate the 20 fields directly to some piece of functionality that the base installation does or does not have, just put that on the purchase screen.
The immediate thought I have is a split screen with the base install on one side with headline: 60% of (Most) users find this satisfies their needs (or something like that), then show the price, describe what it does.
Then, next to the "majority", in a similarly designed fashion, have a simple discreet contact form, with the headline: 40% of users (Others) find it necessary to add functionality specific to their project. Under that, have the form. Under the form have possible reasons (probably the original 20 field form) you might consider customizing the product.
And, of course, if enough people from the 40% pay to have the functionality in there, consider incorporating it into the base install.
For reference, consider Apple's purchase screen. When you go to the store and select a computer, the base models are displayed with the differing specs - then you pick one and go from there. In this case, you would have the base product, specs, and price displayed and let customers know there is a customization option - then I fill out a brief form with my name, email, and a description of my project and why I think the base install isn't right for me.
Of course, if you see your purchases reducing dramatically after making the price of the product/service, it may not be a good idea to publish the price from a business perspective. If that's the case, I would just have a simple (not 20 fields) form for potential customers to fill out and tell you about their needs, and then let the sales team members determine if the base product is all they need.