Ask HN: How are You doing time-effective proposals as a freelancer?

I'm working as a freelancer for a year now and still trying to figure out, what's a time-effective way to write proposals to clients.

I'm doing individual app development (focused iOS) in my DIY build mobile office called 'AppTruck' directly in front of my clients stores/offices. This part is working great so far, because while I'm developing the app in the AppTruck, my client can jump in whenever they want and watch their app grow.

BUT ... writing proposals for an app development is pain, because I haven't found out yet what works for most clients (regarding the proposal). Right now I'm doing proposals individually depending on clients knowledge about apps, their budget etc.

Does anyone figured out a time-effective way to write proposals?