I really struggled to find a reasonably priced maternity coat last year. There seemed to be a lot of competition for them on ebay too. In the end I bought a coat from Primark, just a couple of sizes bigger. I think it was about £20.
It may not be the style you're after but I bought one of these coats off ebay to use as a maternity coat and it is fantastic- the perfect style for a growing bump and a total bargain! http://cgi.ebay.co.uk/Vero-Moda-Ladies-Jacket-Coat -Grey-Cape-Style-BNWT_W0QQitemZ250721203098QQcateg oryZ63862QQcmdZViewItemQQ_trksidZp4340.m263QQ_trkp armsZalgo%3DDLSL%252BSIC%26its%3DI%26itu%3DUCI%252 BIA%252BUA%252BFICS%252BUFI%26otn%3D8%26pmod%3D250 709917569%252B250709917569%26po%3D%26ps%3D63%26clk id%3D4881868719024637693
i've got non-maternity coats, the style that button at the chest and then swing out. One from Zara was about £50 last yr (lost weight since becoming preg so is fitting great even tho im 22 weeks). The other was £25, wool, same sort of style from primark and I bought that 4 yrs ago when preg with DS.
Personally I would look around, you don't need to buy maternity labels if you find the right style of coat.
I second Tesco - £25 for a coat that you can button just at the top. Lots of pretty colours (so much of my maternity wardrobe is black/grey/white, it's nice to have something brightly coloured to wear). I just bought a size bigger than normal and it's doing fine at 27+3 - lots of room for growth still!
I third the cape coat idea - am 24 weeks and still wearing a normal coat (has very loose waist), but I have a cape coat that I got in Topshop 3 years ago and I plan to move onto it as soon as I grow out of this one. I just need to get some nice elbow-length gloves because it doesn't cover my arms to the wrist. I used to have leather ones but one got washed by accident and went all icky. Besides, the cape coats on Asos are adorable!