Campaign group Generation Rent is running a research project into letting agent fees via a comparison website named Letting Fees UK.
The data on tenant fees charged by agents is being compiled by volunteers exploring agents' sites.
The site already lists data for agents in ten council jurisdictions, including Tower Hamlets, Croydon and Manchester City Council.
The project is running as a consequence of legislation introduced last year which requires all letting agents to display the fees they charge to landlords and tenants in their offices and on their websites, as well as make clear which redress scheme they are a member of and whether they offer client money protection.
So far, the website has researched 720 agents. It states that its volunteers have not been able to find public fees for 96 agencies, 13% of those researched.
Of the agencies it has researched, it calculates the average fee paid by two tenants at the start of a tenancy as £391.
According to the website, fees charged by letting agents range from £40 to £780.
Of the 720 agents which have been researched by Letting Fees UK, 33% do not display which redress scheme they are a member of on their website.
Volunteers have been collecting data on agents' website as early as August 2015, just three months after compulsory fee disclosure was introduced.
The highest proportion of agents not displaying fees on their website has so far been recorded in Croydon and Ealing, both at 26%.
Of the jurisdictions which have been researched, the London Borough of Wandsworth has the lowest proportion of agents not displaying fees on their website at 5%.
The site says a 'comparison fee' for two tenants is calculated by adding up all fees charged by the agent that are considered mandatory.
Generation Rent started in March 2014 and last year ran a crowdfunding campaign to try and raise £60,000.