Boards will, nearly always, be cheaper.
So, if you can find a board with excellent grain that will convert to pen blanks with no waste--you got a winner!!
However, straight grained pens are "run of the mill". You want to make "showpiece pens", you need to get burls or wild grain of some sort. With those you will have a LARGE amount of waste. If you buy pen blanks--you have a much better idea of what the pen will look like----sometimes you will still be fooled, but not as often as buying a "chunk" and guessing what the middle will yield. Also, unfortunately, I have never been able to buy a burl that was exactly 4.5-5", so I could get blanks without waste.
IF you are just making "gift pens for the family", straight grain may be fine for you and a board can be a cost-effective purchase.