Absolutely spot on and I appreciate the input! When you are getting the artwork from a customer what program are you using for printing and what format are you asking them to send it in? I am having a tough time at trying to get a process that I can use for graphics coming from customers to be able to use on large banners.
Just request what they're providing up front for new customers, or those you have had issues with. Bigger jobs are less problematic, more flexible, it's the little jobs for small businesses, individuals, etc, that usually don't understand what's needed for printing, who designed it in some off the wall app or free program that looks good on their tablet or phone. I've found most don't have an issue providing it to get a price.
If someone can't/ won't take 30 seconds to email a file to get a quote, what do you think they'll be like to work with? I don't hesitate to pass on a lot of them, if I don't know that I can print it, I can't give a price. If you just eat the time to re-do their garbage and they come back for more, do you keep eating it to keep pricing consistent? Once you see what you have to work with, giving options and more accurate pricing (or just passing) really takes little more time than a ballpark price, with fewer surprises for you on the artwork end, or them on the final price.
File types I'll accept are common vector formats, PDF, EPS, AI, SVG, not flattened. For printing, with good artwork, minor tweaking, converting to any color profile or format that'll work best with your equipment is nothing. Bad artwork you can waste hours and materials on.
If you're doing the artwork, quoting will be accurate, if they're furnishing it, you never know what you'll get. Look through threads here and you'll see the nightmares all shops encounter with the dreaded "customer provided artwork". You'll never eliminate the issue. On the other hand, I have a fair share of steady customers who don't do any of their own artwork, I do it, some I don't even print for. All started out with them, one of their employees, friends, family, or neighbors thinking they were a designer, and costing them more in the end. If those banners are a steady repeat gig, maybe just doing their artwork is an option you can offer them.
Everyone does things different. I grew up in a body shop, we didn't give any kind of price for anything without inspecting, so this is something that's just stuck with me for almost 70 years. This industry is more lax, but you still have to cover your time and overhead whenever/ however you can.