Factors which can affect prices include whether they have their own breakdown vehicles and staff, contract with independent garages to provide the breakdown services, or contract with a large breakdown services provider (eg. AA, Call Assist,Green Flag, RAC, AGA, etc). Contracting with a larger body may mean lower overheads, and thus lower prices.
For example the majority of insurance or vehicle manufacturer branded breakdown services contract with organisations such as the AA, RAC, Green Flag, Call Assist, etc. For example the services offered by Toyota Roadside Assistance, Honda, Ford and Audi are provided by the AA. Startrescue and Gem use Call Assist. Direct Line use Green Flag (which they own). Aviva and Mercedes use the RAC.
Some put restrictions on the policy - such as age or type of vehicles they handle.
The percentage of vehicles repaired at the roadside, customer service, speed of response, etc can vary quite widely.
Buying the cheapest may not always be the best.
For example in the 2019 Which? Car Survey, Toyota Roadside Assistance came top (score 87%l, followed by the AA (80%), Startrescue (75%) and Autoaid Breakdown (74%). This was based on speed of response, percentage of cars fixed at the rosdside, knowledge/capability of the mechanic attending, and value for money.
Toyota Roadside Assistance is £72 annually if paid by direct debit - https://www.toyota.co.uk/owners/roadside-assistance/#/iframe/https%3A%2F%2Fforms.toyota.co.uk%2Froadside-assistance