Iron-on transfer the cheapest I brought is HP 10 (+2 promotional package) at SGD 22.50. which comes to almost SGD2.00 Per A4 sheet.
This is limited to white T-shirts. On other colour T-shirts then be not so good.
Also must remember to design your logo or slogan with clear bold letters... smaller fonts tends to bunch up on printing on inkjet.
Lighter colour also preferred as printout tends to be darker than on screen.
There are also black T-shirts Iron-on... it cost more, nearly twice the price per sheet.
The above is through trial and error
I think with professional T-shirt printers they will do a better job, however the order have to be big else the cost/T-shirt will be higher. Most of the time for clubs, the organisers/admin/perm members will have to decide... make a huge number of T-shirts to lower the cost, then keep the unsold one for future uses.
Eg SW.sg total active members were to order around 20... it is best to do iron-ons... let say if 80 is buying then making another 20 extra to cut down the cost is quite justifiable. The 20 maybe brought by future members or as an extra.