I have built experimental aircraft with AL and organic products (wood) and spent 20 years in the US Navy as a pilot and Aircraft Maintenance Officer. AL is my life!!!
Below is a link to an aircraft supplier's page on what the many types of AL are used for and their numerical nomenclature. From there you can look at their catalog for comparison pricing.
Pages 68 and 69 have the sheet AL pricing with strength and usage information.
BTW if the local supplier is10-15% higher he might be the right person to purchase from anyway, as you may need something on short notice, and if he is still in business, you'll have a local source. You might even have someone to assist you when the bending, forming, etc. operations become more than you can handle without the brake, English wheel, misc tools so necessary in sheet metal work.
Before the flames start, I do not purchase all my stuff from the local supplier, mail order can and has saved me many dollars, but I do use a guide of 20%. If mail order w/shipping saves me 20% or more that's the way I generally go. That said, I have also spent more than mail order locally because of the additional service I received from the local merchant. (8' sheet brake to bend .125" for spars for example)
I hope this is of some benefit.