My IRL Dominion buddy and I have noticed that the shuffler is not including Colony & Platinum very often, now that we have more post-Prosperity sets. Specifically, it includes them 15.9% of the time (figure found using 10% per prosperity card rule and the combin function).
This is too little for us, so we come up with another way to do it. Counting Potion as 2.5, and peddler as 8, the average cost of a dominion card is 4.2. The average cost of a prosperity card is 5.1, and the average cost of the non-prosperity cards is 4.0. You can fuss about my valuations of potion and peddler, but they won't affect the averages across 157 cards much.
Here is our method:
Add up all the costs of your tableau (if you play dominion, you should be able to add 10 small numbers together quickly).
Subtract 40.
Multiply by 10%
Randomly determine if you play with Colony of Platinum based on that probability.
Example: costs add up to 47. Use 70% chance to play with Colony.
A quick simulation shows this increases the chance of playing with Colony & Platinum to about 30% total. The chance of playing with them increases as the cards get more valuable, as intended with Prosperity. Me likey.
Other options:
Don't like the random option? Use 45 as your tipping point.
Like rolling dice?
Use 42, 44, 46, 48, 50 and 52 as your break points, with 1/6,2/6,... 6/6 probability of playing.