Question: Why does X VPS provider charge more for basically the same plan than another VPS provider?
Answer: First, you have to look to see if both VPS providers, provide managed VPS plans or one is and the other is not providing a managed VPS plan.
What is a managed VPS plan?
A managed VPS plan means that the VPS provider is responsible for providing technical support and maintaining the VPS for you. You are responsible for the files you upload and getting your site to work the way you want. The VPS will provide some technical support when you need it when it relates to the VPS itself. VPS plans that are managed will typically cost more than unmanaged plans.
What is a unmanaged VPS plan?
An unmanaged VPS plan is simply what it sounds like. You are responsible for managing and maintaining the VPS. The operating system and software on the VPS has to be updated by you. The VPS provider will only provide technical support when it concerns networking or hardware issues. Everything else is pretty much your responsibility. Unmanaged VPS plans typically cost a lot less than managed VPS plans.
Same technical details or different?
Second, are both plans technically the same thing? Or is one VPS provider providing more memory, processor cores, or more bandwidth? Does one VPS provider include a feature that the other VPS provider does not?
If after comparing the two plans and you are sure both plans are technically the same thing, and both providers are providing the same product (either managed or unmanaged). The reason why one VPS provider may be more expensive compared to another is because that VPS provider wants a higher profit margin. While the VPS provider with lower prices may either be a larger company and can afford to offer lower prices or they have less expenses.
When comparing prices of different VPS plans from different VPS providers. The simple reason on why the pricing is so different may be just how X VPS providers wants to offer a higher quality product while another VPS provider is targeting a different VPS market with a lower price.