A Tanto or any smaller knife can do a whole bunch of things a larger knife can't, so personally I think it's a good choice as a backup to your BAS. Cold Steel makes several knives that would work. For your price range (less actually) the Kobun Tanto is a great deal and scary sharp, although with a thin blade. Kraton handle and kydex sheath. The higher end CS Tanto's in San Mai III are excellent knives and come in about 4 different sizes but are pretty steep. I think they also sell a cheaper recon version in AUS-8.
I also like the CRKT Corkum First Strike Tantos which are incredibly strong convex grind blades, and the most expensive and largest is only about $60. The have a Japanese cord wrapped handle with a black ray skin underlay that is nice, and a multi-position Kydex sheath. I have all 4 models but think the 4.50" blade with about a 9" overall length is best. The blade steel is AUS-6.
Another good Tanto in this price range is the Ka-Bar model 1462 in 1095. It is very sharp with a comfortable handle and black phosphate finish.
The ultimate Tanto's in my opinion are the Bud Nealy knives starting around $275-$300. They are the best. I am saving up for the Southwest Khonvention at the end of the month, but plan on getting either a Bud Nealy Aikuchi in Tanto style or the spear-point Kinzhal with the 9 position MCS carry system sometime this summer. ATS-34 is standard, or you can pay more and get S30V or quite a bit more and get damascus clad ball-bearing steel.
I want to add that none of these are ideal camp/trail knives obviously, as other blade styles are better for that application, although they can do fine in the right hands I think. The Nealy's are defensive knives exclusively.
An excellent choice for the field and my favorite new carry knife that accompanies whatever khuk I am carrying is the Blackjack Grunt. 5 1/2" convex grind AUS-8 blade with kraton handle and a full tang, designed and sold by Ken Warner. About $80, and 888KnivesRus has great prices. It comes with a snap style sheath, but if you go directly to the Blackjack home at Knifeware.com you can buy an excellent handmade pouch sheath for the grunt that is perfect and a great value at only $25. (Buying the knives direct from Knifeware costs you more as I found out after buying 3 knives direct from him and then finding them for less at 888knives. He was still getting his distribution channel set up last year though, so for a while you had to go to Knifeware to find them.)
Anyway, check the Grunt out at:
http://www.888knivesrus.com/product/BJGR
Good luck!
Norm