The expense of it has to do with the fact that it is a high purity medical grade product. It was originally designed for patients with a stoma in order to make a seal for an ostomy arrangement. Its use for breast forms is just a happy coincidence.
Regarding chlorofluorocarbons (from https://www.todayifoundout.com/index.php/2011/10/aerosol-sprays-do-not-damage-the-ozone-layer/:
Despite the lack of conclusive evidence, in the mid-1970s most manufacturers voluntarily stopped using chlorofluorocarbons. Further, in 1978, chlorofluorocarbons were officially banned in the United States, with a few exceptions. These exceptions were primarily concerning certain medical applications, such as with asthma inhalers (though use in inhalers and other medicinal applications were officially banned in 2008).
Other countries quickly followed the U.S. in banning the use of chlorofluorocarbons, including Canada, Mexico, Australian, and many European nations. Thanks to the Montreal Protocol agreement, ratified by 70 countries initially and 196 countries to date, production of chlorofluorocarbons, along with other ozone damaging substances, began to be phased out altogether starting in 1996, the completion of which, even in many developing countries, took place in 2010.
Anyway, chlorofluorocarbons can no longer be used to charge aerosol sprays.
However, one additional thing about Hollister’s is that the forms must be thoroughly cleaned afterwards. If not, any residue becomes increasing difficult to remove. I have used it in the past and it worked well. As I said, it does require diligence in the clean up process. I quit using it because I didn’t want to invest the time and effort to clean the forms.
In the US 7730 can be had for no more than 2/3 of the price you mentioned, and sometimes less. Not sure it would be higher in the UK. Imported product, perhaps?