Let’s assume a 2 story, “walkable” 7:12, 30 year shingles. 60 linear ft. of ridge slot in a single line on a gable (which = 62 lin. ft. of ridge vent, which = a ‘scrap’ 2’ since they are in 4’ sections).
GAF Cobra III is ~ 8.75 per 4' stick. That's approx. 150.00 with tax for 16 pieces.
You’re looking @ 2 bundles of a basic 3 tab ridge cap (I’d never reuse ridge cap in any circumstance that I can think of). Approx. 35.00 with tax for 2 bundles, 75.00 if you’ve got a Z Ridge type cap (12" wide, 25’ long per bundle).
Nails: Cobra III comes with them, but for the shingles (if you’re using a gun like I would) I’d need maybe 5 coil rolls total… no big deal, scraps out of a box). $ 3.00?
Silicone: 2 or 3 dabs for any exposed nail heads. Try to reuse the scraps off a tube. $ 0.00.
Parts estimate: 185.00 to 225.00, total.
Me, I’m not going to give a firm price until I see the site. It’s often difficult to get a customer to wait 2 or 3 days when there’s a long slot open in the top of their house in Wintertime Carolina (& there’s a nice rainy system headed your way again).
So, I go out & look, then run for parts. 1 hour round trip on average. 30 minutes for ladder, compressor, air gun, loading up bundles & vent, plus all the loose tools (hammer, pry bar, hook knife, etc) in my belt. Don’t forget that you’re not only renting ME, but my expertise & the wear & tear on my tools.
Keep in mind that for a 6 or 7:12, you’ve still got a steep enough roof that parts are going to want to slide off. For a total re-roofing, there’s not as much to worry about because you’ve got toe boards & other things spread all over the roof. On a completed roof, you want to limit your impact to the finished product.
Actual time for the labor required is around 3 hours, tops, with one guy. 2 Hours seems possible, but we often price in the worst case, not the least (when it comes to a simple repair like this); probably won’t have to snap lines if the new vent is as wide as the old - you’ll be able to see the shade difference where the shingle was out of the sun. Use this template.
Z ridge is pre-cut, so no slicing 'n dicing required here. Cobra III has nails ‘built in’ so no chasing a loose box or bits in my tool bag.
Clean up - right around the same time as load up (collect parts off the ground that got tossed off on purpose, re-load tools back into the jigsaw puzzle that is an organized truck bed & tool box, re-load the ladder).
Meet with the customer for about 15 minutes to make sure everything meets their approval, site is clean to their satisfaction, etc. Glad hand, give out additional business cards, etc.
In this scenario, you’re talking about 4+ hours, 5 likely. For a solo guy like me, I’m asking @ least $ 650.00 total (P&L) with a 6 year guarantee that you are leak free on the work I did. You just chewed up more than 1/2 my day with drive time to & from the site.
I hope this helps your price decision. Skip a jobsite & see if someone on Craigslist advertises themself as capable in this area.