The website for the U.S. Embassy in Moscow states on one page
“link to this page”
that at the time of the interview the visa application fee is $380 per applicant and that the Form I-864 is to be submitted.

On another page
“link to that page”
it states that at the time of the interview the visa application fee is $100 per applicant and that the Form I-134 (not I-864) could be requested.

(1) How much is the application fee for a K3 Visa?

(2) Is there an additional application fee for a K4 Visa for a child traveling with the K3 parent, and, if so, how much is it?

(3) Which Affidavit of Support, the I-134 or the I-864, is required for a K3 Visa at the time of the interview?

Thanks!

I'd love to help here, but I'm not sure. Try the K-3 forum, a lot of them have good answers on these topics. (Or, maybe when Satellite gets back on here, he'll set you straight, dude is only a degree shy of being an "expert" on immigration, especially in Russia!)

workin4somethin,
Let me try to straighten this out for you.
$100 is for a non-immigrant K3 visa.
$380 is for an immigrant CR1 or IR1 visa.
Are you scheduled for your K3 or CR1/IR1 interview? CR1/IR1 means your wife will be coming in as an immigrant thus no AOS and green card arrives in the mail a few weeks later.
If she does the K3 she pays less at consulate but must pay for the AOS when she gets to the US or she can return to Moscow to do the CR1/IR1 when it is ripe, which in my opinion would be prohibitively expensive. On the other hand, AOS can be pretty slow in the states and a K3 visa holder still needs an EAD to work, but no AP to travel within the validity of the visa.

If you are confused email the consulate and ask. Once you have it figured out, you'll know how much money to pay. Also it might not hurt to ask how far apart the CR1 interview is from the K3 if the K3 is first. If it is not that long, it might be worth waiting for the latter, unless you can't bear spending anymore time apart.

Thanks, Satellite.

We have not progressed beyond the choice of agent on the CR-1, purposely. Thus, the interview is for the K3. I could not differentiate between the two sets of items needed for the interview, but your description clarifies it for me.

We will start the AOS process asap after she arrives here. I leaned towards the K3 from the beginning. And then after waiting through the IMBRA RFEs and a delay in receiving any paperwork once the I-130 was approved, I chose not to spend any more time and money pursuing the CR-1.

I suppose it would not hurt anything for Ksenia to take a prepared I-864 to the interview, in addition to the I-134, just as someone posting here had done. However, we do not want to muddy the water either, with too much paperwork, especially if it is not necessary. Concerning the application fee, it is only a problem for us if Ksenia were to arrive at the interview with less money than is required.