Bad Glow Plugs?

[1Reaper]

So I had the check engine light come on two weeks ago, they told me it was glow plug #1 and #5 bad. This morning I go to work and it wont start, about 25 degrees here in Denver. I let it cycle twice didnt start, I plugged it in and came to work 1.5 hours late

So, are 1 and 5 on the same side? Do I just replace all 8 while in there?

What is a fair price for glow plug replacement?

Thanks!

[1Reaper]

Its a 2006 with 108K miles.