Hello, Yulian, and welcome to chatbots.org! 
Let me see if I can answer your questions. Bear in mind, however, that I’m not an expert with either ChatScript or RiveScript, so better answers may exist. Fortunately, the creators of both of these chatbot platforms visit here on a fairly regular basis, and may chime in here at some point, so you stand a good chance of getting their views on the subject.
What I’m about to relate here is a more or less “worst case” (well “worst” is probably a poor choice of word here, but I’ll keep it for now) scenario, but it’s better to get an understanding of the scope of what you’re asking, right from the start.
1.) How long needed to learn chatbot?
No matter which chatbot platform you decide on (and deciding this depends heavily on what you already know,. though not in areas of programming, necessarily), there will be a “learning curve”. Neither ChatScript nor RiveScript should be difficult to learn the basics of, but both will take time to “master” (i.e. learning enough to be able to write new conversation rules, responses and “gambits” without having to refer to reference material every time). Expect a minimum of 100+ hours of work and practice to gain “minimum competency”, and probably more. Of course your experience may vary, but I feel that this is a fairly decent starting point.
2.) how much it cost to train a bot?
This depends on your exact meaning for the term “train”. If you mean just the writing of responses, conversation rules, gambits and the like in order to have a completely unique chatbot with it’s own personality traits, mannerisms and quirks, then you’re looking at a huge number of hours worth of work, and as we all know, time is money. Just the planning stage for outlining the chatbot’s “profile” (those personality traits, mannerisms and quirks I mentioned) require several dozen hours of time to iron out, and depending on the chabot’s intended “depth” (how complex and varying the responses, conversation rules and gambits will be), could be an “open ended” task, involving hundreds to even thousands of hours. If you’re thinking about hiring someone to write the chatbot’s responses for you, please bear this in mind: I offer both consulting and writing services along these lines for AIML-based chatbots, and my minimum fee is $35USD per hour for just consulting. I charge $50 to $100USD per hour to do the actual writing of the chatbot (I have tools available to me to greatly speed the process, as well as “skeleton” files which greatly reduce the workload). I also offer instruction services at the same rate as my consulting fees so that people can learn to do the work themselves, saving them hundreds to thousands of dollars. The bottom line here is that the “cost” will either br a lot of time, or a LOT of money. For languages other than English, it’s going to cost a lot more, I’m afraid.
3.) I wanted to start with rivescript and chatscripts, are they hard ?
This, once again, is difficult to pinpoint an answer to, as it depends heavily on your knowledge and experience, neither of which I have any knowledge of. Both of these platforms require a certain level of overall competency with general programming principles, such as an understanding of language constructs, branching logic, etc., rather than any specific programming languages (though that may help). However, you also need a firm understanding of the concepts of creative writing. In fact, I maintain that the “art” of chatbot creation is more than 90% creative writing. It’s one thing to have the technical skills required to create the conversation rules and some generic responses; it’s another thing altogether to be able to craft engaging, interesting replies, rejoinders, gambits and queries that not only vary widely (to keep things interesting), but also stay true to the chatbot’s personality. If you ask the other members here about this, I’m sure they’ll say the same thing.
Now if it seems that I’m painting a picture of a monumental task here, it’s probably because I am. This is not something that one generally takes lightly, and many individuals come into this thinking that a few hours of writing or coding will produce epic results, or they may think that this would be an “easy A” for a class project. It’s not either of those, at all. this is something that will require a lot of dedication and effort, but it’s also something that can be unbelievably rewarding, and even fun! If you’re willing to put in the effort, and are not deterred by my words here, then I welcome you to the world of chatbot creation, and will do what I can to help you along the way, as will the rest of the community. Personally, I hope you will accept the challenge. You won’t regret it if you do.
RSS AI Zone
