I fly out of MSP every week so I can help you out!
The Value parking is part of the MSP airport, but it's at Terminal 2. It's pretty easy to park and there are lots of covered spaces. It's at Terminal 2, so if you need to go to Terminal 1 for Delta you can take the Lightrail one stop over there for free (it's no cost between airports). It honestly adds maybe 15 minutes to your travel time. You park, get on the lightrail on the ground floor (I usually park on that floor), then take it one stop to Terminal 1. You end up where the buses come in, so you have to go up some escalators and take a mini-tram to the terminal. Pretty easy and self-explanatory. There's even a sneaky Delta check in counter on the lower level where the mini-tram drops you off.
There are other parking lots as well that are off-site and they generally have a van that takes you into the airport. I haven't really used these so I can't comment. I would assume to add maybe 30 min to your travel time just to be safe?
For Delta if you purchase a few months in advance it's pretty reasonable to find a fare for ~$300. Remember that you also have to pay for checked luggage. I would caution you about the Basic Economy fare. Yes it's cheapest, but you don't get you select your seat in advance (they assign it at the gate). If you're traveling with children they will not necessarily seat you together. I've seen MANY flights where parents are freaking out about this on the plane and trying to switch with people who have purchased their seat, which rarely works out for them.
Let me know if you have more questions!