..................................................................................................................................................................................................... You substitute strength and attack levels for prayer, so actually you are lower CB with 31 prayer then you would be if you didn't have it.
http://osbot.org/forum/topic/26930-gilgads-ultimate-account-making-guide/
"I am going to tell you right now that no matter what you say, getting 31 prayer is more efficient level wise, stat wise, money wise and max hit wise than staying 1 prayer.
Lets say for example, you have an account with 50 attack and 70 strength, you give it ultimate strength (31 Prayer) which is +15% Strength and +10% Attack. This adds +15% Strength to the base strength level (70) which would equate to 9.5 extra strength levels. You would also gain an extra 5 attack levels from 31 prayer, on top of this, you can now use a super strength potion and attack potion.
You cannot gain 15% of your current strength level AND 10% of your current attack level within the 3 combat levels that 31 Prayer gives you, 31 prayer is 100% worth it, if you would like even more explanation, feel free to post below."