I mostly make interior neon signs for the end user -- business owners, artists, or homeowners. So I deal with a lot of people who are on the far ends of their comfort zone in terms of the work that they're asking me to do. They also usually have a vague idea of what they want, but no clear expectations of what it will cost. So my sales conversion rate is pretty low (I'm happy to pass on the "make this crazy thing for $200" customers). As a result, I've put a lot of thought into getting quotes out the door really fast.
My solution on the installation front is to ask questions, and if the answers get vague I just tell them: here's a firm quote for the neon fabrication, but the actual installation is going to be time + materials. Here's a rough estimate: $xxx, based on my hourly rate for on-site work ($xx) and the following assumptions: xxx. Here's a few obstacles that, if present, will increase the price.