Day tours on Saturday or Sunday with Buquebus can be had for about 3200 pesos, or 80 dollars. That sounds hard to beat.
EDIT: That was for Montevideo. If all you need is Colonia then you can do this Monday for under 2300 pesos, with Colonia Express.
The Colonia Express always was the cheapest trip ticket.
Also you may try to travel one day and come back the next.
In Colonia there are several cheap B&B s in which you can sleep over for little money.
The Colonial in Av Flores is one..El Espanol is another