How much should I charge for my garden designs?

[Spike]

I am working in conjunction with a local building firm who recently got a job designing a sensory garden for a primary school. The garden is being paid for by a £6000 government grant the school has recently been awarded, as it is designed for young children including those with disabilities. They asked me to come up with the designs for the garden, and so far I have surveyed the site, come up with two original initial ideas and produced plant and materials lists for them. I will also work with them towards a final design, and will take part in the building process as much as I can. The only problem is, they have asked me for a quote, and I don't know what to charge. I don't have my own business, but I have always worked on designs in the past. The most recent one was a full design overhaul for a pub garden, and I have never been paid for it. I want something for my efforts, but I'm not sure what the going rate is. Any ideas?