How much should I spend on eBay ads

I run a small eBay store in the US selling home accessories, kitchen gadgets, and seasonal items. Sales have been fairly steady this year, but I'm expecting a lot more competition around Black Friday and Cyber Monday. I've been thinking about putting more money into advertising, although I'm not sure how aggressive to get with it. Some of my products have enough margin to absorb extra costs, while others only leave me a few dollars after supplier costs, shipping, and eBay fees. I don't want to spend the busiest shopping weekend watching orders come in only to discover that most of the profit went toward advertising. How are other sellers deciding which listings to promote and how much they're willing to spend during the holiday rush?