I doubt cost had anything to do with it, Wolverine's and Leonidas's were done and are around the same price or less then the T-800.
I also noticed they left out some vein paint work around the neck area that we could see on the prototype T-800.
I don't think you can compare the painted nipples with Leonidas or Wolverine by looking at the retail price, that price is derived from hundreds of factors, like licensing, sculpting, materials used, paints, production time, production numbers etc., so I think leaving the nipples blank may indeed have been a conscious choice, like the veins you mentioned, who actual are visible when the figure is displayed like intended.
I am talking way too much about male nipples here, though. ehm... ****ies!