It depends on a huge range of factors.
On the face of it, it's too little. Â£212.50 a month will get you 3 hours plus change with my agency. Having said that, our strategy will be completely different to yours, we have copywriters, community builders, out reachers, link builders, onsite optimiser, technical devs and so on. All of which have a part to play in the process we use.
I'll stick my neck out and say that you guys are probably doing the same work my link builders would do; going out submitting to directories, do follow forums, link prospecting and that sort of stuff. Given they can churn out high quality links at a good rate, then 3 hours would probably be a enough for that market. But thats 3 hours of one guys time, doing nothing but link building
However, it's subjective and completely dependant on competition, website, website history, social media, brand persona, the service they provide blah blah. Our process ensures we work with high quality clients, stepping away from just rankings and essentially taking control of their brand online. I'm saying all this to show why we charge the amount we do. It's not a case of taking a list of keywords and link building until they're number 1. In order to do that, our starting price (99% of the time) is Â£1,000 per month and thats at a push. We're most comfortable at Â£3,000 (this isn't a dick measuring thing, either. It's apples and oranges because we provide a different service. Just like a design agency charges more then a freelancer. I also think I'm insulting with these sorts of things!)
But back to your question, though; it's too little. But you need to remember that you came to that price for a reason. Charging that much might seem like alot, but thats because thats how much you value your service at. Thats fine, I can't tell you how much your worth to a particular client. I assume you charged that amount because you're new to the SEO/Digital marketing game? Thats fine! But you'll attract the clients that value a campaign based on price, not return on investment.
But I have to ask, what do you provide them for that amount?