Promotion Dollars

I was quite excited this week when I googled my forthcoming book from Medallion Press, The Keepers of Sulbreth, and found the retail catalog they send to booksellers online. I've heard they do a great job with it from someone who works in a bookstore. On my books page in the publication, the promotion my publisher will do for the book was listed. Among the venues listed were the online and print editions of Publishers Weekly. There were a few more places where they will promote my book upon its release. How thrilling for me.
My romances are published by wonderful small publishers who deal mainly in ebooks. All the cost of promotion are my own. With no advance how do I decide how much I should spend on promotion? This was especially true with the very first ebook I had published. I didn't know if my royalties would be in the thousands or the single digits. Yet if I didn't do any thing for promotion, I was assured of low sales numbers. And where could I turn for guidance? Even fellow authors are reluctant to talk about numbers when it comes to expenditures and profits.
Another difficulty is how to measure the effectiveness of my dollars spent. Sure I can check the hits to my website that come directly from a site where I'm doing promotion, but do those hits translate into sales? So far I've refrained from buying ads in major romance magazines like The Romantic Times. But with my first fantasy novel coming out next January, I'm tempted to join fellow authors in an ad appearing in the Realms of Fantasy magazine. Should I or shouldn't I?
It's a dilemma for sure. I wonder how authors in my situation decide what works, what doesn't and how do they decide how much to spend?