Terlor is incorrect. Additionally, your question is slightly flawed. Please read below
There are common misconceptions with Huge Power and Pure Power, and your question already assumes that Huge Power doubles the base attack of Azumarill from 50-100;
For starters, Huge power does NOT increase Azumarill's base attack from 50 to 100. It doubles not the base stat, but the actual stat. Now you may ask lol what's the difference? There's a pretty big one;
Btw using Azumarill ONLY as an example
Lets see here;
Azumarill with NO Atk EVs and a Neutral Nature has an attack stat of 136 - with Huge Power factored in, it becomes 272 Attack.
272 Attack without investment is HIGHER than a base 100 attack without investment - A base 100 attack stat without investment would become 236 Attack.
Azumarill with 252 Attack EVs and a Positive Nature (eg. Adamant) would have 218 attack. With Huge Power factored, that becomes a whopping 436.
A base 100 attack stat, with full EVs and a positive nature amounts to 328. That's nearly difference of 100.
In this case, Huge Power brings Azumarill's base stat technically from base 50 to base 149. Nearly TRIPLES it.
^ Picture proof
Now lets factor in Choice Band.
Choice band works in a similar way - In this case, Choice Band would bring an Azumarill (with 252 Atk EVs and an Adamant Nature) to an attack stat of 654. It becomes equivalent to a Pokemon with base 248 Atk.
If you do the calculations, 149 ---> 248 is a 66.44% increase roughly.
Case 2: Diggersby
With full investment, and a positive nature it reaches 232 Attack.
Factoring Huge Power, it becomes 464 Attack. This is roughly equal to a Pokemon with base 161 Attack (That adds up to a stat of 463 attack - so no attack stat will result in 464 Attack that is fully invested + positive nature).
With Choice Band factored in, it becomes 696 Attack.
According to my calculations, that is roughly equal to base 267 Attack (Full investment, positive Nature). Please note the Smogon Calculator only supports up to 255, so I'm making a guess from the pattern I observed.
So here, Choice Band causes it to go from 161 ----> 267
This is about 65.84%
So to answer your question? It's hard to say, but I'd say it would be around 66% of an increase for Huge Power Pokemon (In the condition where they are both judged with 252 Atk EVs + Positive Nature)