Slightly more than half
Well, it would seem that broadband users are finally outgunning the dial-up users (in the USA at least). According to this article (which is old but I just found it today), slightly more than half of everyone using the 'net in the US is on broadband. From a web designer's perspective, this is good information to know. It means that it's more or less "safe" to put things on your web site that would otherwise be inaccessible to people on a slow dial-up connection.
Even if this statistic is true, I still design my sites to be dial-up friendly. I don't think dial-up internet is ever truly going to go away.. at least not for a long time. I still know a few people who use dial-up.
But check this out:
The cost between dial-up and broadband is almost a wash. The cheapest broadband you can get is $35 a month. The cheapest dial-up you can get is $25 a month.
"But dial-up internet is $9.95 a month." True, for the service only - you didn't take into account the cost of the phone line. Read on.
If you want a plain old dial-up internet connection, you need a phone line. The cheapest phone line you can get is about $15 a month. That's a regular phone line with no frills (not even Caller ID). Add to that the $10 bucks a month it takes to get internet and your total cost per month is $25 a month, assuming the phone call it takes to get connected to the internet is local.
DSL Broadband internet uses the same method, that being the phone line. The line is $15 a month. The cheapest broadband I have found is $19.95 a month. Total cost is $35 a month.
In basic terms, you more or less have to decide whether or not you want to spend the extra ten bucks a month. To be honest, anyone can afford that. Anyone. There's basically no excuse not to be on broadband any longer if it's that cheap.
The only time you would be paying more for fast internet is if DSL broadband is not available in your area. If this is the case, you have to use cable television or satellite. At that point the cost for broadband goes up exponentially, to the tune of around $75 on cable and slightly less for satellite.