An artist's salary would differ mostly from where they live, what's the required amount to have a proper life where your basic needs are satisfied, can pay for services, rent, and possibily to be able to spend on personal stuff or even be able to start a family.
Personally I charge per chapter when it comes to webcomics, by production, not final distribution and meeting a milestone. Same for commissions, I charge per piece and elements inside it (Extra characters, color, background or several changes)
In my country, Argentina, for example a family of four (2 adults, 2 kids) needs around $700 USD per month in order to not classify as poor. (Of course, if both adults work then each would need to make around $350 USD in order reach that number) Still you can live with less, especially if you don't want to pay the "tax income" that basically doesn't allow you to be mid-class either.
I know in other countries you could charge higher depending on your prestige, needs, popularity, and similar, but well, the reason why the market is so diverse is because it is international.
Personally I don't see what would be the problem between charging like an USA or Europe citizen meanwhile being from India for as long as your work, experience and perfomance justifies the price. The concept of being "underpaid" is something that depends mostly on your quality of life and your needs, for someone else making $400 a month could be being underpaid, meanwhile for me that's having a pretty decent salary if you live alone or share an apartment with a partner/friend and have no children.
You can hide where you're from in an invoice, for example Paypal. When you make the bill for the client (I highly recommend avoiding "Pas as family and friends" or "Pay for goods and services", simply have a business account where you can remove data such as phone number, address and mail.
Your clients don't need any kind of information regarding your location, is up to you if you want to share such personal information to a client. Bust most of time, the only ones who seem to be doing this are scammers, since they only do this to make "proper" fake paypal emails telling you you need to pay a fee or some stuff like that in order to "unlock" payment.