It varies depending where I go. At pets at home I've bought plants for £3.49 before whihc i thoguht was steep but I wanted that particular plant and couldnt find it anywhere else.
at my lfs they're 2.99 for potted plants and 75p for bunch plants. cheeky sod once charged me £5.49 for an anubias barteri cos he said it was "rare" but i didnt quibble cos it was big enough to divide and split between two tanks...also because that lfs deserves my money as thanks for their knowledge and good ways with fish.
but yeah I counted to value of plants in my (15 US gallon) tank and it was about £25. what i would recommend you do is get a few cheap fast growing bunch plants, your tank will look empty at first but they will soon fill it up. then you can add other types as and when yo uhave the money.