It was the most fun I have ever had with my clothes on. Shot a friend's the other day. It was a .54 caliber with octagon barrel, double triggers and hammer...not the bolt action type. Huge cloud of smoke, delayed KABOOM..it was AWESOME! I am wanting to get one but I have absolutely zero experience or knowledge of black powder firearms. In fact..I used to scough at them because I had never shot one. He paid $90 for his gun at a pawn shop 10 years ago. What could I expect to pay for an "entry level" black powder muzzle loader similar to his with the double triggers, wood stock, hammer etc...I believe his is a thompson. Thanks guys! It is almost scary how much fun I had. Usually means that not long after...my money starts floating out of my wallet. Any other info is greatly appreciated.

Two to Three hundred dollars will get you a good rifle. As with anything, you can spend a lot more than that.
This time of year, you should be able to pick up a synthetic stock muzzleloader for under a hundred bucks if you just want to get a taste.
Check out Cabelas.
Cabelas Muzzleloaders

Since so many people that just can't or will not learn to shoot a real muzzleloader have traded all of their old style rifles for the inlines you are now able to find good used sidehammer rifles very cheap.

I have seen rifles that were like new sell for one fourth of their new price.

You can now find a good used rifle that will last you a lifetime for about $150

I use a Thompson White Mountain Carbine in .50, altho it's available in .54.  Don't think you can get it with double trigs, tho.  I've seen then used for around $100, $115.  Love that rifle, tho, points and handles great.

Congratulations. Now go find yourself a flintlock.