OK, let me reword that. I'm talking about volume, not actual dosage.
You buy two 10ml vials of tren, one acetate, and one enanthate. The vial of acetate is 100mg/ml, and costs you $100. The vial of enanthate is 250mg/ml and costs you $200. For simplicity's sake, say you are using 500mg/week of the tren in your cycle. For the acetate, you need to inject 5cc ($50 worth) to get your weekly dosage. For the enanthate, you need to inject 2cc ($40 worth) to get your weekly dosage. You inject less volume, and spend less money, than you would using acetate, and get the same results.
Over a ten week cycle, you would spend $500 on the acetate, but only $400 on the enanthate.
Of course I am not taking into account the small difference in dosage from the different ester weights, but you get the idea. Although a lot of sources don't carry tren enanthate, if your's does, even though it costs a little more, it is more cost-effective in most cases, unless you are getting ripped off on the price.