If your book is in fairly final form, there is not much reason not to make it available from Lulu --- it doesn't cost you anything, and you only have to design a cover and reformat the text for a different paper size.
If you want to make it available from Amazon and other booksellers and have it reviewed by MR and Zentralblatt you need an ISBN etc., which costs about $100, and the book will cost more (because the retail price is double the wholesale price).
If you are happy with your copy editing and book design, the only negatives to publishing this way are that libraries are unlikely to buy it(*) and neither your Dean nor your mother-in-law will be impressed.
Despite its $17 price and my hints that I really expect people to buy it, downloads:sales on my Elliptic Curves book are running at about 10:1.
In principle, you should be able to send the publisher your pdf files and they should go into the printer and come out exactly as you sent them. This seems to be how things work with Lulu, but with BookSurge(**) the margins and the colour of the cover were wildly off on some copies, and I had to work through some not very competent people to get this fixed --- my email file has about 100 items. They eventually got it right so this may have been a teething problem. I'll probably use Lulu if I publish this way again, but only BookSurge guarantees Amazon will list it.
Added 2012: BookSurge is now called CreateSpace, and my experiences with them have not improved: be warned. In particular, they seem to be adding a page of unauthorized material to some of my books, and they are not responding to emails (the only emails I receive from them are junk). I won't be publishing this way again.
(*) A search of WorldCat.org found about 40 libraries owning Elliptic Curves
compared with about 400 for Etale Cohomology.
(**) BookSurge is now called CreateSpace, and is owned by Amazon.