Hi
Your project seems relatively straight forward, but it all depends on the pedagogy; level of difficulty you want, variety of skills included and/or combined skills. I cannot give you a fixed price, but to give you an idea, I normally work at about £40 per hour. I've bid £500 but I would could not give you an actual price until I have a better understanding of the project. It might be more expensive than most freelancers, but I have the experience, results and qualifications to merit this:
I trained secondary maths before moving into primary where I now teach year 6 and lead in Maths and Computing. Each year I work as an examiner for the KS2 SATs and last year led a team of markers; I tutor KS2 - GCSE; I review educational apps and act as a consultant; I design my own content and create questions for rote learning and specialise in differentiated automated resources. I am am now positioning myself to move into an advisory role hence broadening my range of work, but rest assured, any jobs I undertake, I complete to the best of my ability.
Incidentally, I've used Freelancer for a while to create my own projects but haven't bid on any. Yours is the only one that has really interested me so far. Finally, I can offer up to 10 hours per week, but would struggle to undertake more than this.
If your interested, let me know.
Neil