Well it will depend upon how they are printed.
The cheapest will be screen printing in 1 colour, gold for instance.
The down side is they will have to be done all in one hit so some one will have to fork out a lot of money up front.
However screen printing in two colours requires two screens so doubles the printing price for small runs.
OTOH direct to fabric printing can be done in any colour that your colour laser printer can print and Gold is not one of them.
So the colour would have to be different.
However most print on fabric mobs will allow 4 colours for the same price as 1 colour.
Dye sublimation is very cost effective as you post out the transfer and the recipiant irons it onto their own shirt.
This is what most of those t-shirt booths use or used to use prior to direct print to fabric.
Again you are limited the the Gammut of a laser printer.
Die sub needs a shirt that is at least 30 % synthetic fabric
Direct to fabric requires a 50% cotton ( or hemp ) fabric
Screen print works best on coton but now days they can do 100% synthetic