My first interaction with a provider found on BP was by email and I was asked for a 50% down payment before any further communication (through a cash, electronic means). Is this common or even to be considered?

A fool and his money are soon parted.
Plenty of ways to have a bad experience without sending cash or equivalent to a Backpage provider/
Not normal at all.

Do a search here on deposits. This topic is asked often

Deposits have become increasingly common.  

If the provider incurs expenses to visit you - getting an expensive hotel room, flights/gas/taxi, time invested...she can ask for a deposit, to ensure that her time is not wasted, and the time/money she spends on hotels/flights/etc. is not her loss. She can ask...and you can also refuse, if it is not in your comfort zone.  

Personally, I'm not getting a room at $300+/night (plus gas, plus parking, plus incidentals)...with no deposit. If I don't get a deposit, I won't feel comfortable making an appointment. I'd be having nightmares the entire week before, worrying that the client won't show up, and that I'm out money that I really needed to use for something else...

BUT, I've been in business under my current name for 2+ years. I have an online history, and no complaints against me...I've invested time and money in my persona...I have reviews and regular clientele... so I'm not going to steal a guy's money and disappear. That would cost me much more money than whatever he'd give me for a deposit.

But you really need to do your research on the provider.

If she's just a BP advertiser, with no reviews, no website, no online history...I'd say pass

...TER. Use the TER regional ad board for your area.

My thoughts no deposits are that there are very few times a deposit should even be considered. If she is traveling in just to see you or you are booking her for a long period of time, and she has a long history with many favorable reviews and you can afford to lose the money if she decides not to see you, you might consider a deposit.
 While there are many trustworthy ladies there are also many who look at a deposit like an easy payday from a guy she never even has to meet.

What could possibly go wrong here?

Please don't EVER send someone you've met on the internet anything...EVER!!!!!

Use TER to research the women...read their sites and reviews.  I trust you've bought VIP here...if not..do so!

There is never a reason to advance anyone anything for any reason.  If the woman is simply seeking an advance for whatever reason...find another (and there is NO shortage of women who would NEVER ask for a deposit).

I hope you don't reply to random emails asking for wire transfers as well?  

Sure!  Just tell me where to send it!  No, I don't respond to unsolicited contacts.  But, this "ad" pictured such a beautiful lady that I was intrigued.  And, again, this was my first interaction with a provider and I have yet to meet up with someone.  But thanks for the advice.  I think the VIP thing is a worthwhile investment for me.

I work in conjunction with my brother who is a Nigerian Prince.  Our father has amassed a veritable fortune and we have a need to work with nice unsuspecting souls like you to transfer our funds to the US.

I would also guess that the pic on the ad you referenced is a fake.  Use google to run a scan on it...I suspect you'll find that it is fake.

Educate yourself BEFORE you find yourself out money.  And even more important, find yourself in a place that might be even more damaging.

Read this board...and go back 10, 20 30 pages until YOU can recite the most correct way to evaluate who you are trying to see.

When can I expect to get paid?  This is a limited time offer...and we're counting on you.  If you act now I'll make sure this gal (see below) can entertain you  LOL

This one is easy, a resounding NO!!!  Many providers will require a deposit for an extended date, esp, as VooDoo pointe out, if she has considerable expenses to accommodate your date.  If she is a high dollar provider, has plenty of reviews, and asks for a deposit to guarantee you will show up and she is not out whatever she spent in preparation, a deposit might not be out of line.  

Personally, I have never given a deposit, or even been asked for one.  Have not had many high cost dates either.  I would have to be very sure of her to give her money ahead of time, or I would just move on.

LLAP,
Swim

She wants 50% before further communication.  Kiss that money goodbye.  At that point, she's done no work & has your money.  Why would she actually do any effort for only half the fee?  

Does she have reviews?  Have you googled her phone number?  Tried searchingher on TER?
Have you used Tineye or Google images to see if the pictures are really her...  or stolen internet images...  

Even on BP, girls get reviews...

never give a deposit to a BP escort at all...reviewed or not.

I'd carry it further to all escorts, but there are scenarios where it's legit.

END OF MESSAGE

If you choose to proceed, either she'll cut off contact and keep what she got, or you'll show up and find that it's not the girl you were expecting. From there, you can expect to go through a lackluster session since "you're already there and had paid," or she'll try to upsell you even more.

Winwood, Clapton and Baker. Goo stuff.