Race entry isn't cheap, especially for marathons. However, occasionally, if we know where to look, we can find races that are surprisingly inexpensive. With the help of volunteers, generous sponsors and a bit of good fortune some race organisers are able to provide marathons that don't break the bank.
Here's a list 10 of the cheapest marathons (or marathon series) that we could find. All prices are from the past 12 months (as of June 2020), for non-affiliated runners.
1. Malvern Midsummer Marathon - £15
Organised by LDWA (Long Distance Walkers Association), this trail race takes runners on a tough 26.2 mile journey over the Malvern hills. 
Prizes / swag : There is no medal or race shirt, but all finishers receive a certificate. Entry includes a soft drink and hot meal at the finish line.
Location : Malvern, Worcestershire
URL : http://www.ldwa.org.uk/heartofengland
2. Cambridge Boundary Run Marathon - £16
Organised by Cambridge University Hare & Hounds (The University Cross-Country Running Club), this marathon follows a route around the historic Cambridge boundary. Incorporating some footpaths and trails, the route is 30% off-road. 
Prizes / swag : All finishers receive a medal, but there are no cash prizes.
Location : Cambridge
URL : https://cuhh.soc.srcf.net/boundary-run/
3. Town Moor Marathon - £21
North East Marathon Club are the team responsible for this event. A traffic-free race, on footpaths through and around its historic Exhibition Park and Town Moor, it is a perfect example of their work to provide affordable, no fuss, scenic, local running events. 
Prizes / swag : All finishers receive a race medal and a hot drink and snack is provided to all runners.
Location : Newcastle, Tyne-and-Wear
URL : http://www.northeastmarathonclub.co.uk/town-moor.php
4. Strathearn Marathon - £24
Strathearn Harriers, a popular local running club with a reputation of hosting good races, is responsible for hosting the Strathearn Marathon. It takes place over quiet B roads in Strathearn.
Prizes / swag : Each finisher receive a medal. In 2020, to reduce the entry price, no race t-shirt was included. Race shirts were available to order at a cost of £10.
Location : Strathearn, Perthshire
URL : http://www.strathearnharriers.org.uk/our-races/strathearn-marathon
5. Hellstone Marathon - £29
Badger Trail Events are the husband and wife team who, with some help from friends, organise this event. They are also responsible for several other local trail races.
Prizes / swag : Medals are given to all finishers, and hot drinks and refreshments are available to all who cross the finish line.
Location : Little Bredy, Dorset
URL : https://www.hellstone-marathon.co.uk
Flipbelts : Different colours, sizes S/M/L, £27.99
6. Cybi Coastal Marathon - £30
Organised by local residents, the this marathon circumnavigates the island of Ynys Cybi on the Welsh national coastal footpath and Anglesey coastal path.
Prizes / swag : Finishers medal and t-shirt.
Location : Holyhead, Anglesey
URL : https://cybicoastalmarathon.co.uk
7. Boston Marathon (UK) - £30
Although not to be confused with the US event, this road race is a popular local and national event.
Prizes / swag : Medal, t-shirt and goodie bag
Location : Boston, Lincolnshire
URL : https://www.bostonmarathon.co.uk/
8. Cape Wrath Challenge Marathon - £30
One of the races making up the week-long Cape Wrath Challenge. This locally organised road race takes runners on a scenic and challenging route.
Prizes / swag : A medal will be awarded to all finishers.
Location : Cape Wrath, Sutherland
URL : https://capewrathchallenge.co.uk/index.html
9. Running Grand Prix Marathons - £32
This series of marathons at 3 different race tracks, Oulton Park, Goodwood Motor Circuit and Bedford Autodrome is organised by Runthrough, who also provide numerous 5K, 10K and half marathon events throughout England.
Prizes / swag : Medal, race t-shirt and goodie bag. A free massage and race photographs are also available.
Location : Oulton Park, Goodwood Motor Circuit and Bedford Autodrome
URL : https://www.runninggrandprix.com
10. Isle of Man Marathon - £33
One the few events to take place post-lockdown in 2020. Organised by the Isle of Man Veteran Athletes' Club, the marathon is a 2-lap road race starting and finishing in the island's capital, Ramsey.
Prizes / swag : A medal will be awarded to all finishers.
Location : Ramsey, Isle of Man
URL : http://www.isleofmanmarathon.com
Note: We are confident that the above list includes 10 of the least expensive marathons in the UK. However, if you think we've missed a deserving entry, please don't hesitate to let us know and we'll make sure to give it a mention.