^I agree. I guess the quickest way to get a pool of potential is what the artists themselves are willing to pull out. Kind of like an auction for a job. Lowest wage with the highest quality wins (Or some variant thereof). You can always bargain with certain offers if the pay is a bit higher than you can afford, but at least you get to see
all the possible options than just the options limited to your budget. Keeps an open horizon for ya in the future
There isn't many professional mangakas in the US, much less those with coloring. Because of that, prices tend to be quite steep. If you were hiring from Japan, it'd be much easier to do so, as they have an entire section dedicated to manga work, but then you'd have to account for yen-dollar exchange, language barriers, timezones, etc. etc. (Honestly I wouldn't hesitate to jump on a US version of that, if there was one...
)
Page or hourly depends mostly on what the artist and you agree upon. Hourly would be more effective if you're looking at fetching out the most amount of pages in the least amount of time, but that also means lots of active work on both parts. It's very loose in what you can accomplish. Page is more set in stone as you are assured the artist will do that many pages, but that also means you can't go more than that and could constrain you if your story stretches beyond that limit.
I don't think I need to go over writer expectations, as you very know well your capabilities
but for artists, keep in mind the time and effort it takes to make the first draft of their work. First the lineart, which can range from a quick 5 minutes to perhaps an hour based on changes or rearrangments, then there is the details, which takes a good 1-3 hours or more, then there's the outlining, the shading, the polishing, coloring, etc. And all that for one panel of a page. It's nice to know where a realistic work boundary is, so you don't accidentally trap yourself in a pit of delays and fallbacks due to some unforseen circumstances (I always give a lax schedule with the group I'm working on for a one-shot. You never know when real-life hits ya
)
Most importantly, get to know the person you're working with on a personal level. Compatibility and positivity are the best motivators for any work, even more so than cold-hard cash!
If the people you work with are pleasurable to be with, perhaps you can set yourself with to create more and even greater pieces of work with a trustworthy and reliable network of compatriots.
Good luck with your search, and I wish you great success in your endeavors!