Quarterly Plans Instead of Monthly?

I have reached the plans & pricing stage with my new startup and am considering quarterly plans. I know monthly & yearly plans are the norm so I am hesitant to step away from these. But the situation may lend itself to quarterly better...

The startup, twitMate, is half way between SaaS and standard service. Because, for each user, there is a significant amount of work at first but after that it basically runs itself.

I want to avoid an extra 'setup cost' as this will complicate the build and I am aiming to keep things simple for users too.

The setup work...

It's a Twitter management service and I will manually hand-write text , find images and select hashtags for Tweets as well as set up a individual API access for the user's Twitter account.

I estimate about 5 hours work per account to setup. After that people will have the option to edit things themselves and our tweet and follower engines will take care of the rest.

The cost of my time...

Price-wise, I'd like to cover the cost of my time in the first payment so something in the region of $200 / €175 / £150. However, this feels too expensive per month and looking round the market confirms that there is some competition that costs about half this (although some that is really expensive too).

The plans...

So I am thinking of quarterly and annual pricing - off the top of my head something in the region $400 / €350 / £300 a quarter and $800 / €700 / £600 a year?

What do you think?