Noizchild, I work as an intern for a small editing company, so I'll give you my perspective. Hopefully it helps!
I'm honestly not sure if you want a developmental editor at this point. If you're looking to expand the plot significantly, that's something a developmental editor *can* help you with, but it is going to be pretty expensive, considering that you're then going to write and revise a lot. So you'd be paying for 45k words to be edited, but depending on your editor's policies and how much you add, they might not edit the new stuff without additional costs.
It sounds to me more like you might want to find some new readers, or ask the people who've already read it, and find out what parts of the story could be fleshed out more. A good critique partner is usually willing to be a sounding board for that kind of thing.
As to how much developmental edits cost, it depends a little on what level of editor you go with. I know quite a few skilled, established writers who also do freelance editing, and their fees are based off hours/page count and the EFA's standard fees. As Maryn said, that's $45-55/hour for developmental editing. Many freelance editors offer different packages, so you might consider someone who does manuscript consultations instead. That's when you talk over plot issues, character arcs, or issues with other critiques you've had, that sort of thing. A paid sounding board, if you will.
If you go with an editing company, like the one I work for, that's going to run you more. It depends on the size of your manuscript, but I've seen costs ranging from $500-$2000. I work for Kate Foster Editing, so I can only tell you what our practices are, and assume other companies are probably similar. One (or more, which doesn't cost you extra, we just sometimes like to get more than 1 opinion on a MS) of the pro editors on staff will do a thorough edit. We make line edits and copy edits as we go, because we're all obsessive and don't want your work to have errors, as well as providing feedback on scenes and overall arcs. You'll get in-line edits in the manuscript and an edit letter, which goes over pacing, characters, arcs, formatting, audience, genre suitability, word count, and anything else we think you might need. We give our authors resources on how to fix any problems we find, or improve writing skills we think need some TLC. And we encourage our authors to keep in touch with us as they revise as much as they need and we'll read revised scenes. We're thorough, encouraging, and in it for the long run with your manuscript--and that's also why we cost more.
Investment-wise, I'd say you probably should consider finding a good critique partner to talk things over with or a freelance editor who does manuscript consultations. If you want names of freelancers, I can share some I personally know and would vouch for. Good luck with the novel!