Great information!
If you already have a 30* bag, and are happy with it, I would use that. Depending on when you plan to arrive in the Yellowstone area (looks like you might arrive before June so verify openings), the 30* might be a bit cool. However, if you have 2 people in the same tent and put the tent under trees (under canopy is about 5* warmer than exposed), that might work. Once you get into mid-June, you should be OK. Worst case, throw a rain jacket or something over the bag while you sleep. Oh, and wear a wool beanie at night! Another suggestion is to consider a bag liner. It can add up to 15* of warmth and you can always mail it home once you know you will not need it anymore.
Look at WeatherSpark.com to get a good idea as to the AVERAGE climate for various places along your route, especially those early in the tour and when at elevation. You can see what the average lows are (and various percentiles) in a given area so you can decide which bag temp is good for you.
By budget, I meant budget for a bag

. If you are buying a different bag, I would highly suggest you buy a USED Western Mountaineering bag, preferably one without side baffles. Those without side baffles allow you to shake the down to the top or bottom as temps warrant. Reason I sat used is that WM has a fantastic warranty. The bag is warrantied for life, regardless of who was the original owner. Plus they make excellent bags. I personally shoot for sub 3-pound 20* bags without side baffles but I get cold at night. I like my WM Megalite since I am not 25yo & slim but would consider the Alpinlite for even cooler temps. Remember, the degree listed is under optimal conditions so plan on about 5*-10* higher temps than listed.
I would get a good dry bag for your down bag (unless you stuff it into a waterproof pannier like an Ortlieb). Regardless, if you do get the bag wet, be sure to dry it completely often as a "funky" smell can occur if you put a damp (even from normal human perspiration) bag into a dry bag for days on end. Same goes for drying the tent out frequently.
You should also consider a sleeping pad that has some insulation or at least reduces the cold ground from cooling the mattress.
Sounds like you have a wonderful tour coming up. Tailwinds, John