If you want to take a course for certification then the price was around 30k two years ago. it is 2500 to 4000 for single flights - you get a better deal if there are more people with you..
In terms on flights, 54 days ago was a magic number but this is changed over time. Now, the prices are totally fluctuating depending upon various factors such as season, time of flight, demands, destinations tourism demands and lot others. So now there is no such fixed time to book, however mon to wed is the best time to book. Same applies in terms of hotels. Also avoid over searching for tickets, it leads to cache traffic which companies utilize to inflate prices.
Well it's all about your preference. If you like the mountains you can head to Kashmir, Dalhousie, Dharamshala or somewhere in the North East like Sikkim or Cherrapunji.
My personal choice for a trip in April would be to the Andaman Islands. It's the best time for Scuba Diving and other water sports.
In North Goa you can opt for sinq beach or vagator beach which are comparatively less crowded than other beaches in North Goa. rest in South Goa, most of the beaches are less crowded
Hey! I am going to Char Dham this year (2019) and I have made my booking with Bizare Expeditions (bizarexpedition.com). They were ready to customize and make a personalized itinerary for us after a long trail of emails. We went ahead with this organizer after comparing around four to five organizers. I can share the experience after I come back from the trip. Best wishes for your trip!