Ive really looked into this, and for my 1.6 Astra Design they quoted me £995 including a hire car as they need it for roughly 2 days. This being said you have to be quite careful as they should provide a certificate to prove they are legit, approved LPG installers. Ive seen quotes from £995 the lowest, to £1699 the highest.
Its not cheap to have done, i had often thought about a conversion but to gain/offset the cost againt the difference in price means you'd need to keep it for a few years. I would suggest you find one already converted, or possibly a kit out of one thats been scrapped & get someone to fit it.