Number plates with two letters and one digit sold in the 90s tended to go for between £2,000 and £10,000.
But the same plates can fetch up to £45,000 now, according to Jon and based on data from the Driver and Vehicle Licensing Agency,
Jon said: "We're seeing investors come out from the fine art markets and wine investors becoming interested in number plates - because it is proven to deliver returns.
"People who get into it make money quickly, and it becomes very addictive."
Jon Kirkbright is the founder of Plate HunterJam Press
Jon said he recently sold personalised number plates to an England and Arsenal player - and recently sold one plate for £140,000.
The DVLA releases new registration plates twice every year, on the 1 March and 1 September.
There's also a thriving trade in vintage personalised plates, Jon said, with investors often buying and reselling rapidly.
He added: "There's lots of ones which started in 2007 and 2010 for three, four grand.
“By five years ago they were 10 grand and now we sell them for 20 grand. It"s just supply and demand. There's people who pay and that's the thing."
Unique number plates can fetch even higher sums with ‘F1’ - for Formula 1 - having sold for £440,000 in 2008, and ‘1D’ - for One Direction - having sold for £285,000 in 2009.
The ‘F1’ number plate belongs to businessman Afzal Khan, who has reportedly refused offers of more than £10 million for it.
Jon, 45, of Burntwood, Staffordshire previously worked in car sales, and then sold car insurance.
He listed a few number plates he found on eBay on his website 11 years ago.
He now hosts 60 million plates on his site, including registrations released by the DVLA where he acts as a broker.
He said he "can't keep up" with demand from well-heeled investors.
One of the things driving the boom is cash-strapped councils selling personalised number plates which used to be used for senior local government officials.
"In the past decade or so, there've been a lot of councils selling their personalised plates which were gifted to them in the past," said Jon.
"The councils are selling them to make money after being gifted the registrations back in the day."
This has meant that a lot of desirable numbers have come onto the market - with East Renfrewshire Council in Scotland selling the rare ‘HS 0’ number plate, and saying they hoped Harry Styles would buy it.
"People want number plates that are their initials, or their girlfriend’s or their daughters - ideally with a '1' on the end," said Jon.
He said that people sometimes do try to buy "rude" plates, but this can have unintended consequences.
One buyer, who had a number plate with an extremely offensive word with a '1' in the middle, bragged about his purchase online.
Jon said that the buyer seems to have been contacted by the DVLA, as the social media posts rapidly vanished and the registration is no longer on the road.
He estimates that one in three cars in the UK has customised plates - with people often buying plates to make their car look newer, or older, if they don't want neighbours to know they bought a brand-new vehicle.
"Back when I launched this website, my mum said that wouldn’t get any clients, because personalised number plates were only for the rich," Jon said.
"Now when I look at cars on the road it seems to be one in three."
Divorcing couples sometimes buy new plates so their former "other half" can’t keep track, Jon said.
Jon said that while there are bargains out there, sellers should watch out for unscrupulous dealers promising high prices which people won’t pay for.
He said: "Some of the bigger dealers will hoodwink you and say it’s worth more just to get you on the books."
"There’s no set price. I’ll usually have a conversation with sellers about what's sold or what’s available at that price - and what the plates originally sell for, to give the customer the idea.
“Then the customer said ‘Yes, this is the price I want’.
"Then we advertise it on the site - it’s all about having the right price. You can"t just pluck figures out of the sky, you need to look into each one."
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