What we do is work to their budgets and we usually ask what budget they have up front - so we have a £5k offering up to £50k with large increments and a portfolio of what each would look like and we show them. They choose what they’d like and that is the point where you know what their budget can comfortably manage.
If they are very constrained on the budgets and want high production standards then RT may not be right for them, they would be better with normal arch vis. However if the RT side is a big issue then production polish can become a secondary issue or the budget will be found.
In our experience the larger the project the more RT can add value so you may find through the life of a project that you start with a rough and ready version and basically work to milestones bumping up the production standards until the required polish has done whatever job/selling it needed to do.
Developers are 100% business men through and through so the main advice I could give is stand your ground on price and if its squeezed don’t budge or add features and boost the price, they’re only fishing for a best deal and will use every trick in the book to squeeze back their budgets.