We do our tunnel at least once a month, possible more on rainy days with shovels and a wheel barrow.
We just had a company come out to clean out our self serve bays. 7 bays and two overflow man hole tanks were $850.00 flat fee. Previously we had the same company out in May 2017. Some companies do charge by weight. This one does not. If they do, pump out all excess water and leave them the solids to keep costs down. Get a few quotes as these prices are pretty random.