We've done a lot, and we mean, a lot of advertising.
Hands down least expensive is Project Wonderful. Yes,it has its flaws, but it is super easy to use and super affordable. Plus 70 percent of every penny you spend there supports webcomic artists and that's something important, keeping resources within the community. Depending on how good you are at finding your target audience and how patient you are with bidding, you can get clicks for a sub-penny. For us we have gained 10,000+ clicks for $300, so $.03/click.
Next up is TopWebComics. The site gets about 500K monthly visits so it's fairly well visited and one of the few mainstays in a community with no real hub. They have several ad options, the best in terms of price is their $1 Daily sponsorship, which ends up translating to about $.035/click for us.
Last is Google AdWords. It can be a real beast to learn and tame and if you aren't careful with your budget, it can make a lot of money disappear very quickly at some real expensive click rates. We really micromanage it and after a lot of trial and error finally got it to come down from $.09/click to around $.038/click. However because your ads are reaching a broader body of people, you're going to see lower read thru rates than PW and TWC. However if you want to spend a lot of money in a hurry and get a massive hit of views, then Google AdWords is king. In theory you could almost spend an unlimited budget with them.
As far as Tapas advertising, the company has only so many resources at its disposal so it has to decide, which is better, building the cruise ship or advertising cruises? To date the focus has been just building the ship, putting in a framework that can really maximize the most from readers participating such as the tipping program. Once they have reached a position where they feel like everything is present then they will probably start advertising.
In terms of a webcomic, we highly recommend having at least 25 pages available before starting ads, otherwise you are spending money trying to reach readers who arrive to find too little to read.