When I was looking for a Certifying Body for my company's SQF certification, I narrowed our search down to three potential CBs. I interviewed them and requested quotes. There were HUGE cost differences between them. One of them was more than twice as expensive as the other two and had extra days for the audit that the other two didn't have, just because it was the CBs policy to add extra days to everything.
I think trying to find a general cost range is not easy because every company has different needs and every CB has different costs. You may need to make a list of potential CBs and contact them. Additionally, we have different CBs for our Organic Certification and SQF Certification; however, I believe there are some CBs who certify both standards.
As for year to year cost for attaining and maintaining certification, those have been pretty similar for my company for the three SQF audits that we've had. The fluctuations were due to the variable costs of airfare, hotels, and rental cars. I believe this also holds true for our Organic Certification, I'm not as involved in the cost side of that certification.