Does anyone know the price of this potion because im unsure at this point and im just selling them for random prices to my players!

Xanathar's Guide to Everything provides a table that gives estimates, under the Downtime Activities section, with the Buying Magic Items section. This is an uncommon, consumable magic item which would mean (1d6 * 100) / 2. So between 50 gp to 300 gp,

Does it change your base stat to 21, or your total stat to 21? My base is 15, but thanks to being half orc, it's raised to 17.

It changes your total stat to 21, just like giant belts would. They negate your base calculations completely. Keep in mind that with a belt for example, if your new strength stat would be 21 and you use an ASI to raise your strength, it would become 17 + 2 = 19, not 21 + 2 = 23. Giant belts and potions overrule your current stat if the stat of the item is higher.

So... do you have to drink it or can you rub it on your skin? There's a nail in there. I'm giving my players a flavor option. Rules as written is nasty.

Quote from ViscountRoller >>

So... do you have to drink it or can you rub it on your skin? There's a nail in there. I'm giving my players a flavor option. Rules as written is nasty.

 Bottoms up! Gotta drink that toenail, it's the secret ingredient 

Would Dispel Magic nullify this effect?

Quote from AlliedVoyd >>

Would Dispel Magic nullify this effect?

 Not from rules as written. Dispel magic should really be dispel spells. It works on spells (with a few exceptions that specify dispel magic won't work). It does in very rare cases work on magic items to stop them temporarily or something like that, and some constructs get stunned by dispel magic. Dispel magic does work on any magic item that lets you cast a spell though, but it stops the spell not the item. 

By RAW, dispel magic wouldn't do squat. But by RAI and any DM worth his weight, dispel magic would dispel this effect if it's already drunk. It would have no effect on it until it is drunk and actually active.

Quote from Vrizsemoz >>

By RAW, dispel magic wouldn't do squat. But by RAI and any DM worth his weight, dispel magic would dispel this effect if it's already drunk. It would have no effect on it until it is drunk and actually active.

 I'd disagree with that, both because mechanically there would then be no recourse, it's just "haha, you lose out on your potion use, sucks" especially since there is no spell level for the potion, so therefore no save needed to turn it off. But secondly, it also implies you could also do stuff like cast dispel magic to stop a healing potion from working, which is silly.

Besides, if a DM feels the need to house rule in a counter to an item like that to stop the players from using it, they should have just not given out the item in the first place.