The short answer is YES it is bad practice to use the $ symbol in URLs - . As an alternative, you can encode it as %24 but the best option (for SEO as well) is to use the word 'dollar' or 'dollars' - for example: www.pizzahut.com/vouchers/50-dollar-gift-voucher.

For the long answer, read on.

The dollar symbol is a reserved symbol within URLs - it should not be used as part of a URL except for it's specific technical purpose (as a sub-delimiter). Any attempt to use a reserved symbol in a URL will either break the URL or result in the reserved character being encoded, in this case the $ symbol would be encoded as %24 making your example www.pizzahut.com/vouchers/%2450-gift-voucher

While there are parts of a URL that may contain the $ symbol without breaking the URL, it's safest to avoid them except for their reserved use.

For more information on reserved characters, you can read the URL specification here: http://tools.ietf.org/html/rfc3986#section-2.2

Using currency signs in URLs can negatively impact SEO for several reasons:

1. Non-standard characters: Search engines prefer URLs that are easy to read and understand by users. Currency symbols are non-standard characters for URLs and can cause confusion or encoding issues.
2. Poor user experience: URLs with currency signs can look spammy or untrustworthy to users, reducing click-through rates.
3. Encoding problems: URLs can only include certain characters safely. Others, like currency symbols, need to be encoded, which can make URLs look messy and less readable.
4. Internationalization issues: Including a specific currency sign can limit a page's appeal in international markets where a different currency is used.
5. Inflexibility: Pricing or currency changes would require URL changes, which can lead to broken links and loss of SEO value unless properly redirected.
6. Difficulty in tracking: URLs with special characters can sometimes cause issues with analytics and tracking tools, making it harder to measure the performance of specific pages.

For better SEO, it's recommended to use simple, readable, and descriptive URLs without special characters. Hope that helps.

I wouldn't use $ symbols in URL structure as Micha mentioned this can break the URL and cause issues.

Look at how most of the big Coupon players such as Retail Me Not set up their sits they have a category level page for "PizzaHut" and they list all the deals on the one page they do not set up seprate pages usually and if they do they dont use $ symbols they use dollars as micha mentioned.