Noob question: transaction costs

[Mediocre_Player]

So far, my investing has been limited to Index fund purchases, with a long term strategy. However, now I'm looking to get into ETF purchases (and possibly individual stocks)

My question is: what is an acceptable % cost incurred for stock/ETF purchasing. At what point does dollar cost averaging become unattractive b/c of the money lost doing multiple transactions?

ie. I have X dollars available to buy an ETF. How do I determine if I'm best off just using all the available money up front.

Hope this questions makes sense.