Here is my screen shot explanation of what is going on.
(for those not sure what or why I am listing this, check HERE.)
This is our starting point. Note the starting model count is 5.
Once you get that then add your psycannon, it will add an additional model. Hence making the total squad model count to 6.
Now go back to the model count to 5 by subtracting one model from the top line
Now after we have done that the total cost is only 25pts higher.
Any questions?