In the case of PPC, in order to test your landing page, you first need to settle on your Pay Per Click terms and ad copy. There's not much point changing your PPC target phrases *and* the landing page at the same time while in testing phase - it will be too difficult to gauge the effectiveness.

Get traffic flowing to the page to the level you want, then tweak the copy. When editing your marketing copy, do it step-by-step; adjust, test, adjust, test. Too many changes as once can be as ineffective of making no changes at all.
A good general initial test is to ask friends and colleagues to view the pages for about 5 seconds. Don't tell them what the page is about prior to displaying it to them. Allow them to view it and rather than ask if they thought the page is "good", ask them what they remember about the page - their concept of the message you are attempting to relay. If it ties in with the question mentioned earlier of "what are you offering", you're on the right track.

Use Google Analytics for statistics on how long people are viewing your page for - if it averages just a few seconds per visit, then it's obviously not grabbing their attention.
In the case of PPC advertising, each click is costing you money - sometimes dollars per click.

The best way to increase conversions and reduce your COA is to ensure that you have a well-honed landing page focused on the terms that brought people to the page. Put in the effort, don't throw away your advertising dollars by directing people to a general product page or your home page and you will reap the rewards.