There are two separate issues - exam fees from the RHS, and charges by exam centres.
The fees charged by the RHS for exams are designed to offset the costs of writing,moderating, marking and re-marking exam papers etc . The RHS doesn't get money from publishing its
qualifications, syllabus,past papers etc., which anyone can use to teach a course in horticulture. The exam fees (£42 total for each level 2
Certificate) to cover running the exams are all it gets from students.
Less than ten years ago the RHS Certificate was only offered once a year in March, one LONG exam, but people complained about waiting a whole year if they missed the day or had to resit. Splitting the exams into bite-sized chunks offered twice a year gave students the chance to take them bit by bit as it suited them. The overall fees haven't changed much for the exams, but because there are more exams being run it does have an effect on the costs charged by the people who give you the opportunity to take them - the exam centres.
The fee set usually results from an administrative decision to try to recoup some of the costs of exam invigilation from external students - their own have already paid a lot more, perhaps £4-500 per certificate, for their courses to cover all these things.
Not many colleges take externals at all now, and I've even heard 'Not our students, so not covered for Health and Safety ' cited by one college exams officer as a reason for not taking anyone.
£25 - £30 is probably going to be the general cost from a college for entering you for the exam and running the exam under the required secure conditions. After all, these are internationally recognised professional qualifications in
horticulture, so exams and conditions in which they are taken are subject to a lot of regulation.
If you aren't happy with this you could ask the Examinations Office about 'Exceptional Supervision' arrangements to take the exam somewhere else, but I don't think it would work out much cheaper if you have to pay for invigilation and a venue.
It's the unfortunate down-side of being able to spread the study and exams out, I'm afraid.
best of luck with your studies anyway,