I'm confused on bag prices, does it mean the first bag is 30 and if you bring a second bag is it 40 for both or is it adding 40 to the 30 which makes it 70 for two bags?

Airlines determine bag prices, not the airport. Ask the airline you're flying. Southwest charges nothing for 2 checked bags. Allegiant and Frontier charge for a carry-on. It varies.

Back in the day, you could store your guitar in the cockpit. Now you must check about everything. Your calculations are correct. 70 to 80 bucks but it could go up depending on the airline.

Cost varies and depends on the airline

Which airline would determine this. For example, American allows only 1 personal item if it's a basic economy ticket. If it's a budget airline, it's usually cheaper to pay for bags in advance.