Overclocking the Base line clock is a terrible idea as it messes up PCIe and memory timings. As for the cost of a Z87 motherboard for overclocking, any will do. With Haswell the speed you can get is dependent on the silicon lottery a helluva lot more than the board you are OC'ing on. So a basic Z87 board will overclock to the same speed or damn close to a £200+ board.
I still maintain that a £30 investment now is very worth a possible doubling of the life time of your Mobo and CPU.