Correct... It's not really worth bothering at all if you are putting less than 20k or more as the odds of winning anything at all really fall off a cliff the less you have invested.
Bang it into a isa or even a good standard savings account for a guaranteed consistent return.
This free tool delves into the Premium Bond probability to reveal the real odds of winning based on the bonds you have. Plus tell it what you've won in the past and it shows whether you're Mr Luck or have had a black cat cross your path. Developed with help from post-doctoral probability...
www.moneysavingexpert.com
With 15k invested in PB for example, you can expect to win £500 per year with average luck...
Whereas a 4% savings account would yield a guaranteed £600 in a year from 15k....