I get a huge 20 liter bag of aspen for like $8 and a brick of eco is $4.
1/4 bag of aspen and brick of eco mixed put about 2 inches of bedding in TWO 20 longs, 50 gal reptile, AND 55 gal.

So if you are doing it in only one tank and break of 1/4 of eco, you should get 4 to 6 months before you have to buy more.