Expect the taxi from KLIA to Crowne Plaza to be around MYR100. Train is MYR35 each to Sentral and taxi from there is going to be around MYR30 for a premium taxi.
If you are really adventurous you could lug you bags from the KLIA Ekspres to the Monorail station at Sentral (down the escalator, across the open lot through the mini markets and over the road) and take the monorail to the Crowne Plaza for about MYR3 each.
The train+taxi option will be quicker, but not a great deal depending on the time of day. The taxi is probably more convenient in that its only one vehicle. But plan on sitting in it for 45 mins or more.
Taxis at KLIA and Sentral are on a pre-paid basic. Follow the signs to the taxi (Teksi) service desk and tell them where you want to go and they will tell you the price and give you a receipt/voucher. Hand that to a drive and off you go.
I can't remember at KLIA (only used a Teksi once from there), but at Sentral there are two desks, one for regular taxis and one for premium taxis. Premium taxis cost more, but the cab is larger, newer, clean, the driver is likely to speak English, and the air-conditioning is likely to be working! Worth the extra money in my opinion.
I always use the train now since I stay at the KL Hilton which is right at Sentral and very convenient for me.
Note that for your return journey, you can check-in at Sentral and hence don't need to lug your bags out to the airport. You can only check-in for Cathay Pacific, Malaysian Airlines and Royal Brunei flights at Sentral. You must be there at lest 2 hours before departure for them to take your bags. You could plan to get there 3 hours early, check in, then spend some time looking around the shops and markets around Sentral. Then hop the train out to KLIA. I always check-in there and find it very convenient and efficient.
Enjoy KL. Its a great place to spend some time.