My shop produces hundreds of software project estimates for everyone from high-impact entrepreneurs to multi-billion dollars companies each year (and probably half or more of those are mobile specific.) It's not uncommon to have clients request these sorts of deliverables upfront to help in their evaluation and selection process.
Generally speaking I would agree with the previous posters on it being fairly standard to charge something for producing mockups (in particular using the example of discounting what you would charge towards the bid price of the eventual full project; only being collected if they decided to move in another direction.)
However, I will play the devil's advocate here just to offer a contrarian opinion:
When done properly, producing a mockup for any project (particularly a mobile application) takes into account things like researching the target end-user, studying competitors and UX patterns, the integration of market/industry best practices and even mapping out your entire user flow (information architecture, API integrations etc.) However, purely as a UI exercise, just creating a mockup of a few screens should in and of itself being a straight forward process. While this does not mean you should put a limitless amount of research/design time into the project before being “on the clock”, it does mean that you should be ready to accept a certain amount of sunk-time for each project you onboard. What I would recommend, in order to not let this get out of hand, it to come up with a formula to track/monitor if you are allocating an appropriate amount of time to the business development opportunity. Let’s say, for example, you are willing to spend 1 free hour of work for every $2,000.00 of opportunity the project represents: if the total bid/budget from a client is somewhere in the $40,000.00 range you should be ready to spend no more than 20 total hours on business development and related tasks (in this case, creating mockups is part of that total.) This should at least give you a sense of how to scale your Biz Dev efforts beyond just this one opportunity moving forward.
The bottom line is if the opportunity is something that is game-changing for your venture (i.e. larger clients are far less common for you currently), now is the not the time to quibble over a few hundred dollars. The focus you should be spending is on making a high-quality mockups and value-added exchanges to help land that new business. I’m available for a call to discuss estimating, bidding and business development tactics with you further. Good Luck!