I hope you don’t just send a prospect a single price in a proposal and then hope they say yes. Not sending options is going to cut your earnings and gives your prospects a binary decision. They can only say yes or no. If you take just a bit more time and dig just a bit deeper with a prospect you can offer options (3 options is best) which turns the binary yes/no decision on to a yes/yes/yes/no decision. They're then deciding how much value they want to engage with you on.
Even when you offer options in your proposals the lowest cost option should still meet all the base level requirements of the project. Doing anything else is unethical.