Client side only ( aka, if your game has no server ), wont affect your hosting cost in the slightest between Flash and SWF.

HTML5 is certainly the future, while Flash is certainly more capable. The biggest flaw with HTML5 is audio handling, which frankly it sucks at. Most audio libraries actually fall back on Flash when it is available. I've recently done a tutorial series on Cocos2D HTML5, a JavaScript port of the popular Cocos2D game engine, and the experience was mostly a positive. Unless you have advanced audio requirements, HTML5 is probably the way to go. Use a library though, let someone else deal with all of the various browser bugs, of which there are a freaking ton. As to picking a library, this list should get you started.

Of course, Flash would work too, but Flash's future on the web is remarkably questionable. In fact, just this week Adobe pulled the Flash player from the Android market place, so Flash on Android and iOS is effectively dead.

so going with HTML5 is best ???

It isn't that simple; what's better, orange or lilac?

Is HTML5 a good choice, yes.


As to your how much, again, it isn't that simple.

If you are just serving a few static webpages, you can get by with a cutrate ISP that will charge you between 0$-10$ a month.
If you need more control, or want server side access, you need to go the dedicated or virtual dedicated route, which tends to be more in the 50-100$ a month range.
If you want a full blown server, you will pay about 100-300$ a month. Of course options like Amazon EC2, Softlayer, etc exist
This is just the tip of the iceburg, and again is way premature.


Also, 100 active viewers is nothing, will cost you nothing and you can get by with the minimum of everything.
100 concurrent viewers, that's a different story. That is, handling 100 requests at the same time.


That said, get a free/cheap host, write your game, and worry about these details later.