I reckon to do a decent job with sequencial multi point injection etc £2-2.5k depends on tank location and size. A cylinder in the boot is probably cheapest but pointless (why lose the boot space) a new custom built small petrol tank(25l) and twin cylinders in the space created is probably the most expensive.
much better to try and find one converted already. Doesn,t add value to a used car so should cost the same (ish) to buy a converted one as not. shop around tho...