It would depend on what you were advertising... if the people who use facebook are your target audience, then you will get better results, if you are selling zimmers for people aged 60+ i can't imagine you would get any return...
Give it a go, if its your target Market, then why not? Start low on £250 maybe, and test it to see if its any good, if not, its not that much money wasted, if it is then you have found a good method for marketing your product.
Just make sure you can track how many orders you get from your ads. then compare this with how much you spend to get those sales and work out a cost per sale.
Thanks for your advice, keep getting mixed feedback but i think, like you say, its down to whther or not the product will benefit the facebook user.. i was thinking of around £250 on facebook and the same on google to see waht happened.. ill let you know the result when i get around yo it!!!