Yes, you will need to affix a daily max to each ad campaign that you create- for example 5 dollars a day max, etc. Although that sounds really high, so far my campaigns have really cost me peanuts, probably because there are only a handful of people competing against me for space.
I seem to be getting some pretty good bang for my Ad Commerce buck, although I don't see any real dramatic sales increases to date (it's kind of tough to measure how much real impact Ad Com has had on sales, but it has certainly driven a ton of traffic to my store as you will see below).
I list on .ca and my ads include five campaigns- two in Canada, two in the USA, and one in the UK.
Here's a look at my total numbers since April 8, 2009 (These ads took me about 30 minutes total to make up and I haven't touched them since - I have just left them up and running, and once in a while I check in to see how much it's costing me- it's been so long, I probably would forget how to create a new one! LOL)
TOTALS:
Impressions (number of times its been seen): 15,751,437
Clicks (number of times people have clicked on the ad): 12038
CPC (Cost Per Click): $0.01
Avg CTR (Click Through Ratio): 0.08%
Cost to date from April to Oct '09 : C $179.33
So for 180 bucks for approx. 5 months, that's pretty cheap advertising, and money well spent, I think.
Cheers,
AXE