^single digits, double digits and tripple digit numbers are usually on advance booking. the singles usually go on auction and can fetch silly prices.
however since the series' change so quickly you can get single and doubles pretty cheap (7-15k) ut usually the singles go for more,... usually when a new car/shape is launched.
i'd suggest that if you can't get the number you want in the current series, just book the same number in the next series, it's alot cheaper. i got my "MA" number 2 months before the series was launched at auction but luckily there was only one more bidder for it so i got it at a reasonable price. till then i drove on a temporary IDF number (it's fully legal to do so as long as you have the reciept of th eauctino number (which i kept in the car).