I don't think I'd ever want to do without one after having one for the last few years. If the unit is good quality I don't think the price is too bad - if it includes the labor.
I installed a RVS-770613-213 some 5 years ago on my Roo 21SS. (Got it from Amazon for less than $280 - it's $300 now) I installed it myself and it worked beautifully from day 1 until the day I removed it - just before I traded the 21SS in this last Oct.
I had planned on re-installing it on the new camper but I found out during the walk thru that it came pre-wired (power) for a wireless camera from Furrion. (Funny what you miss when you're checking out a camper trying to decide whether to buy it.)
We ordered the High-Speed Observation System (#FOS48TA-BL). I thought it was a bit pricey at $500 (minus a one-time 10% discount) and the monitor is a lot smaller but it was extremely easy to install and so far it's getting the job done pretty nicely.
2016 Roo 23IKSS / 2011 Tahoe
2011 Roo 21SS / 2011 Tahoe
2006 Bayside / 2006 4Runner
1999 SantaFe / 2001 Sienna
1967 Nimrod / 1998 Outback