There are a few resource spots you can consider, but I would strong recommend just sticking with what they offer directly on their site:

https://www.twilio.com/sms/pricing?v=7
https://www.twilio.com/voice/pricing
(VIP or offbook pricing is a little harder to nail down since they are going to make a specific offer to you depending on volume and just how VIP you are haha. I've seen certain companies get as much as a 25-50% discount on top of the book prices for at least a short period of usage or after they meet certain volume requirements)

As for the integration side, your search is going to depend on what type of connection you're trying to make. Integration with mobile? Integration with various 3rd party platforms (Salesforce etc)? Regardless, Twilio does have extensive API documentation available which helps developers understand if/how to best connect your platform and theirs.

Doing a feasibility analysis for your idea could be as easily resolved as reviewing the Twilio message boards/documentation, or I've seen it require (in certain cases) an actual prototyping phase for both validation of the connection being made and to better scope out an accurate cost of the functional scope.

I've helped 50+ new ventures over the years develop planning around their software needs including feasibility planning, platform diligence, functional specification documentation, project charters, cost analysis, business plans and fundraising pitch decks. If you'd like to connect to discuss/answer product-specific questions for Twilio integration or other product dev best practices feel free to reach out and suggest some times!

Twilio has their pricing on their website: https://www.twilio.com/pricing

I'm not sure what you mean by VIP pricing, but no one can give you a definite answer on Integration unless they know about your business model. Integration into an existing app via plugin will be far less expensive than paying a developer or team of developers to develop an app from scratch with the Twilio API integrated into the code base. It all depends on what your goal is, and if there are solutions already available for the problem you are trying to solve.

Contac their sales team: https://www.twilio.com/help/sales