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My Venezuelan girlfriend wants to travel to the USA to be with me. How difficult and expensive will it become and what should she do to attain this task?

First, she needs to get out of the country. https://ve.usembassy.gov/visas/ says:

On March 11, 2019, the Department of State announced the temporary suspension of operations of U.S. Embassy Caracas and withdrawal of diplomatic personnel. All consular services are suspended. Immigrant visa applicants should contact IVBogota@state.gov for additional instructions. Nonimmigrant visa applicants can apply at any U.S. embassy or consulate in another country. For more information regarding applying for nonimmigrant visas, please visit travel.state.gov.

Then usual procedures apply which include proving ties to her residency, including family, jobs etc. If she resides in Venezuela, at this time I have serious doubts she'd be able to prove this. There's no formal ban, just a huge amount of suspicion about whether the visitors are genuine.

In fact, according to some news reports the US last year already was revoking tourist visas and another article also said

Venezuelans say they’re already seeing a drastic reduction in the number of U.S. visitor visas they’re being granted – and that the visas they have are often being revoked.

If by "be with you" you mean marriage and you have the funds to do so, I very cautiously would recommend getting her to Peru or another visa friendly country (at least it seems those flights are still operational) and apply for a K-1 visa or even getting married there and returning to the United States as a couple. Researching these options are far beyond the scope of this answer, I am afraid.

But I have a few ideas: I said Peru above because has a USCIS field office and it's the field office which handles such affairs for Venezuela. This might be beneficial or it might not be depending what you file and where. Also, looking at other field offices I can't see any other places accessible by direct flight where she could stay for six months without a visa. And here's a blog post about foreigners marrying in Peru. So: maybe Peru. Maybe. Don't take life altering advice from a random stranger on the Internet (like me). Lawyers who can help with this will be costly but a misstep might cause months of delay (or worse, ruin the whole thing). The process will take 1-2 years most likely.

But if you want more advice from internet strangers, marriage questions belong to expats.