What card game is popular today, fun for kids, and is a worthy investment at a reasonable cost to my wallet?
Collectible Card Games (CCG) wax and wane in popularity....

For example, both Pokemon an Yu-Gi-Oh! cards were extremely popular over a decade ago. There are still sales of these cards and older cards sell well, but both games have shrunk considerably. 

The exception:
Magic: The Gathering which has had pretty solid sales, organized play, and card values over the course of the last 2 decades. If there are game shops within your area, chances are very high that they sell both new cards (decks and booster packs) as well as buying and reselling cards. There will also likely be organized tournaments and play events at these shops as well.

The Catch

Magic (and CCGs in general) can be described as money pits and aptly so. The pre-built decks you can buy work, but not nearly so well as building a deck using rares and uncommons found in booster packs to fight a specific strategy (or to counter one). They have yearly or quarterly expansions which drive players to continually buy new cards as well. Magic card values also have the usual collectable game catch-22 where they are only worth something if you never play with or even physically touch them which makes collecting for value something of an adult hobby vs. for your son. 

The Solution

In recent years there has been a huge trend in Living Card Games (LCG). LCGs work on the same basic principle of deck vs. battles as CCGs except they completely do away with boosters and have a growth and expense rate of expansions for a boardgame. Two very popular and very well reviewed LCGs are The Lord of The Rings LCG and Android: Netrunner LCG. 

Android: Netrunner LCG is a 2 player game Cyberpunk dueling deck game with 1 player taking the role of the Corporation (there are different ones, as different decks) trying to complete agendas and make money who is taken on by the other player playing the Hacker (again, different faction decks to choose from) who is seeking to hack into the corporation and expose agendas for money or for fame. 

The Lord of the Rings LCG is a cooperative game where players take the role of different heroes from the Fellowship as the travel through Middle-Earth. Players work together to deal with enemies and dangers as they are dealt out. Everyone wins and loses together.