This is always a tricky one! There is not (as far as we know) a public price transparency mechanism that would enable you to check this
A lot of people end up just using online retail to check the current price (coles / woolworths; ceres for organic) and then adjust for wholesale with a standard mark-up across all. I found a ref from a few years ago: 47% mark up average wholesale to retail on all fresh food. This is of course only gives you an extremely rough guide but could be a starting point.
Then of course need to take into account that you are not really comparing apples and apples in this comparison. The wholesale price is based on bulk amounts of product, with fairness to the farmer definitely not the driver, and your small organic growers probably cannot and should not attempt to match it!
So it is important to sense check/adjust this based on a bottom-up assessment of costs / break-even. Here is a resource to help with this process:
and in more depth from rodale institute:
Also see this great resource from Manchester Veg People in the UK - see from page 23 re pricing. They worked with their farmers to properly calculate fair prices based on costs (see appendix for detail). They then calculated a fair service charge for the food hub based on its costs and then they made it all transparent to customers:
These resources may be interesting - some price comparison work that Monash students did for us a couple of years ago looking at food hub prices against Gippsland supermarkets:
Grow Lightly Research Note.pdf (428.1 KB)
Baw Baw Organics Research Note.pdf (388.0 KB)
South East Food Hub Research Report.pdf (2.4 MB) (SEE PAGES 10-11 for pricing audit)