Ever fancied owning the Joker's costume from Batman?

Some film memorabilia fetches millions of pounds at auction, but it can cost nothing to start a collection.

A life-size replica of the Joker, as played by Jack Nicholson in the 1989 film Batman, leers down from a podium. His plum-coloured suit is unmistakable in its sinister glory.

A few metres away, a mannequin sports a coral and maroon-hued cowboy outfit that looks like it's seen better days.

It once belonged to fictional character Marty McFly and was worn by Michael J Fox in the 1990 film Back to the Future Part III.

It's the day of the Prop Store's memorabilia auction and at the BFI Imax cinema in London, some of the film world's most recognisable props and costumes are on display ahead of a sale that afternoon.

Also up for grabs are model miniatures of the Los Angeles skyline used in the making of Blade Runner, the 1982 sci-fi classic.

And there's a pair of Garth's "tighty whities" underpants from the 1993 comedy Wayne's World 2.

While some items will go on to fetch relatively modest sums, others, such as a helmet worn by Chris Pratt in recent superhero film Guardians of the Galaxy, will sell for more than £100,000.

So what is it that compels punters to spend a fortune on film props and costumes?

As chair of the David C. Copley Center for Costume Design at University of California, Los Angeles (UCLA), Deborah Nadoolman-Landis is well placed to offer an opinion.

She's been a costume designer for more than 40 years and her work on Coming to America, the 1988 comedy starring Eddie Murphy, secured her an Oscar nomination.

In the industry, she's affectionately known as the "Mother of Indiana Jones" for designing the outfits worn in Raiders of the Lost Ark.

"People are supposed to fall in love with the people on screen and when you fall in love with Indiana Jones you want something from the character. Memorabilia is an extension of falling in love with the film."

It doesn't stop at cinema, she adds. Ms Nadoolman-Landis also designed the costumes for Michael Jackson's 1983 Thriller music video, which "everyone was obsessed with" at the time.

"Michael's red jacket ended up being sold for $1.2m (£900,000)," she says.

But not everything in the world of memorabilia costs the earth, says Jon Baddeley, head of Bonhams auction house in the UK.

Granted, Bonhams New York sold the piano from the film Casablanca for $3.4m, and Mr Baddeley hopes to sell a Robby the Robot prop, used in the 1956 classic Forbidden Planet, for seven figures at an upcoming auction.

But he says it is quite possible to start a memorabilia collection for free.

"The film posters and lobby cards made to advertise films in cinemas often get thrown away. So why not make friends with your cinema manager and ask for posters or cut-outs when you see a film you like?"

So could today's rubbish be a future collector's item?

An original poster for the 1933 monster adventure film King Kong can fetch around £70,000.

"Who knows?" says Mr Baddeley. "But remember, whatever you buy you've got to live with it. Have it framed and enjoy it. If it goes up in value you've got a double whammy, if not, you've still got something you enjoy."

At the auction, Stephen Lane, the head of Prop Store, gives me a whistle-stop tour of the day's top lots.

He won't be giving anything away for free but says that, in amongst the stratospherically expensive nuggets of movie gold, there are some very affordable items.

"The lots start at £40 to £60. For that you'd be buying crew items or gifts, things like call sheets which were used in the production. It might not be quite as personal but it's still something from your favourite film."

That's good news for first-time auction visitor and Blade Runner fan, Chris Dagger.

"I don't have a big budget, the most I can spend is around £500," concedes Mr Dagger. "I've got my eye on the Blade Runner crew jacket. I've always been a fan of the film and with the new one coming out, I'd love to buy it."

The jacket in question is dark maroon satin with the name "Tim" stitched onto the front.

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"I don't have much chance of getting it," says Mr Dagger. "There are people here with a lot of money to spend, a lot more than I've got, but we'll see."

In the imposing auditorium of the BFI Imax the auction gets underway. Things don't look good for Chris.

A single grey T-shirt, embossed with "Peace through superior firepower", which was worn in the film Aliens, is sold for £3,000. And a full alien creature costume from the same film fetches £50,000.

Chris holds his number "26" bidding card nervously, waiting for his lot to be called.

The auctioneer announces a starting bid of £250, and within seconds Mr Dagger is pushed to his maximum bid of £500 for the Blade Runner jacket.

There are no further bids in the auditorium, but there's an agonising wait as the auctioneer checks whether other offers have come in by phone or email.

None have, and to his delight and astonishment, Mr Dagger gets what he came for.

Out in the refreshment area, he takes a couple of deep breaths and poses for photos next to a poster for Blade Runner 2049, the film's soon-to-be released sequel.

"It's such an iconic film and if you're a fan you know all the trials and tribulations they went through to make it.

"And here we are, surrounded by posters for the current film. I'm just really happy, there's not much more I can say."

With the 22% buyer's premium, VAT and shipping charges, the final bill is £700, which doesn't make the jacket a cheap purchase.

But Chris says it was definitely worth it.