As Mansur said, the fees are based on the different services the HOA provides. For example, I live in the Keys off of Civic. My HOA bill is around $500/mo. But that includes servicing the 3 pools, 4 tennis courts, the clubhouse, the common grounds, the common heating and water bills that the other 792 units use. Also that includes a discounted cable rate for all the units (I think it is $16.00/mo for all the cable channels, not including HBO, Showtime, etc, but the normal rate for a house would be much higher). And also the PG&E for the unit is based on the unit's square footage. They can't the monthly bill for the building and divide by the square footage of each unit.
If you look at some of the other units in the WC area, some have concierges or other amenities that the HOA covers. Also other places have higher rates to cover future assessments (i.e. replacing the roof, or HVAC systems, etc). Think about it, if you had a low HOA due of $100/mo, but had to pay $5-10K in assessments every other year to cover major renovation projects, or lawsuits, which would you prefer to do. I would recommend that whatever place you go to purchase, read through all the HOA minutes and CC&R documents. In the minutes, you may find out why the rates are what they are, or at least find out what the major renovation projects are, if any.
I live and work in Walnut Creek. So if you need any assistance, please feel free to contact me.