If they are a hard coral they are not what you want. The hard corals are nocturnal, hard to keep (must be hand fed) and reported to have a stong sting (I never noticed the sting but avoid them for an octo tank). You would see a cluster of very identifiable hard tubes but not likely any of the orange or black polyp heads. If they are a cinnamon brown with white striping (the amount of white varies with age and lighting but there would be at least some white mixed with the brown) and no hard tube, these would be the ones I am referring to.
Your LFS is likely to have mushrooms and the red ones Nancy referened grow quite large, are hardy and octo safe. Most mushrooms are ok but avoid any called ricordia (multi colored and quite attractive for a reef tank) as they have a definite sting even though you will not see long tenticles. Fortunately, most octos will learn quickly to avoid softies that sting if there is plenty of room to roam without bumping them.
If they have an affordable leather coral of any kind, it would be very acceptable, hardy and you can often find them in a nice display size.
I forgot to mention Kenya "tree" coral (a soft coral). It likes light but is a nice pinkish tan, grows rapidly with requires almost no care.
$5 is not at all bad. If you see coraline algae (purple), bits of orange, macro algae (not hair algae) and/or tiny hard white tube when you inspect the rock, the price is excellent but more likely it will be lifeless on the outside. Be sure you allow the rock acclimation time before adding your new octopus. Wait at least a week and watch your ammonia an nitrates and wait for zeros. There is almost always a slight cycle time even for cured rock. Look at all the rock in the water and see if you see small (pea sized or smaller) semitransparent polyps (they will disappear when disturbed). If you can avoid getting rock with these (aptasia) it is best to do so. You won't always see them if they are there but in some shops the rock is loaded with them and erradication is all but impossible, control difficult. If you see dime sized or larger (majana anemones), I would avoid rock in the tank altogether. Both forms of nusience anemones sting, are not a disaster but do have to be controlled. Joe's Juice will control the aptasia very well but the majana's are more difficult. I have found that keeping a peppermint shrimp in the tank (and have had some success keeping one with an octo if it was there before the octo) will control aptasia but not kill the visible population. Multiple peppermint shrimp can pack and become agressive so I recommend keeping only one in a 40-50 gallon tank. If you try to add one after the octo is in residence there is almost no chance it will survive more than a day.
PS forget the growth additives. The most common is Purple Up. If you want to add anything for new corals the only thing I will recommend is a vitamin complex with high B's. I do add vitamins to my reef tanks taking Martin Moe's recommendations (from one of his books - he is one of the pioneers in marine aquariums and is still somewhat active in the hobby and in reef restoration). Even this I use sparingly and don't have a favorite brand.