Book layout is a very specialized field. If I were writing a book of my own, I'd save up the extra money for a layout person, particularly if the book had a lot of tables or illustrations, or if it used a lot of non-standard characters.
Rates for layout are currently anywhere from $45 to $85/hr, so the cost of laying out a book depends on the length and complexity of the manuscript, if there are any non-text objects, and your editor's experience. Book layout is definitely a non-trivial task, and a not a quick one; it's essentially another edit pass, with focus on how the text will appear and flow. Doing some quick calculations, the cost quoted by your editor seems to be quite within the realm of reason.
If you still can't afford the layout, tou might consider getting some competing bids for the job; any professional editor will have no problem with that. But consider that this editor is familiar with the text, and can work more quickly than another editor who'd have to start from scratch.
If you want to do the job yourself:
One thing to consider when choosing a layout program is what kinds of files you'll need to generate. However, your question about marketing seems to indicate you're going to use either Lulu or CreateSpace:
As to whether Scribus is a viable alternative to InDesign, this question seems to indicate that there may be some issues. For example, it's mentioned that creating high-res output is finicky, and there are problems with styles and tables. Kind of like using an older version of GIMP instead of Photoshop, or maybe more like using Inkscape instead of Illustrator.
This isn't surprising, since it's an open source program. But it may be usable for small or simpler projects. If you've been using Scribus, I'd give it a shot and check the output very carefully. (You should do that in any event.) If it doesn't look good, all you've lost is time.
I'm not sure if Scribus can directly handle the epub format; as of 2012, the feature was still lacking. I'll keep digging and update this if I find out more.