Experienced flyer advice needed!!!!!!!!

Do I buy miles or not?
I signed up to the virgin frequent flyer miles. I was going to get the virgin double miles card but, it was withdrawn. You can buy miles i.e 2,000 for £45.00. 2000 miles would give me discount on a flight to Orlando with part miles part cash.
But, what sort of price would the ticket be and would a £45 investment pay dividends?
For example a £500 air fare would cost, HOW much? £300 or simply £500 less my £45 investment i.e £455?
Can anybody tell me if its worth buying miles and what sort of discount could I expect for milesplus cash.
Thanks
I signed up to the virgin frequent flyer miles. I was going to get the virgin double miles card but, it was withdrawn. You can buy miles i.e 2,000 for £45.00. 2000 miles would give me discount on a flight to Orlando with part miles part cash.
But, what sort of price would the ticket be and would a £45 investment pay dividends?
For example a £500 air fare would cost, HOW much? £300 or simply £500 less my £45 investment i.e £455?
Can anybody tell me if its worth buying miles and what sort of discount could I expect for milesplus cash.
Thanks