Since Roman Abramovich took over the club in 2004, Chelsea have been renowned for splashing the cash in the transfer window.The Blues have made a plethora of mega signings over the years, and they rank among the richest clubs in the Premier League and across Europe.Top ten transfers in Chelsea history:Under the Russian billionaire's reign, Chelsea have broken their transfer record six times, as they often break the bank to acquire the brightest talents across Europe. Didier Drogba was Chelsea's first record signing under Abramovich after the Blues paid a hefty £24m to snap him up from Ligue 1 outfit Marseille.The next season, Michael Essien arrived from Lyon for £24.4m before Chelsea splashed the cash in 2006 to acquire Andriy Shevchenko from AC Milan for £30m.The list goes on and on. While some of these mega deals have worked out for the club, others have turned out to be underwhelming.On that note, let's take a look at the ten most expensive transfers in Chelsea's history.#10 Andriy Shevchenko | £30m (2006)Ukraine vs Sweden - Group D: UEFA EURO 2012Following a stellar seven-season stint with AC Milan, Roman Abramovich went all out for the Ukraine international in 2016, smashing the record fee Manchester United paid for Leeds United’s central defender Rio Ferdinand.However, despite scoring 23 goals in 77 outings for Chelsea, Andriy Shevchenko failed to live up to the hype surrounding his move. While he often fell behind Didier Drogba in the pecking order, at 29, his experience did offer quality to the Chelsea attack.Shevchenko eventually returned to AC Milan on loan after two years at Stamford Bridge before the move was made permanent a year later.#9 Mateo Kovacic | £40m (2019)Chelsea vs Manchester United - Premier LeagueIn 2018, Chelsea snapped up Mateo Kovacic from La Liga heavyweights Real Madrid on a one-year loan deal. However, after being imposed a 12-month transfer ban, Chelsea opted to make Kovacic’s deal a permanent one.The Croatian put pen to paper on a five-year deal with the Stamford Bridge outfit after a £40m clause in his loan contract was triggered by Chelsea.Mateo Kovacic vs Everton (H) - Pass Map:94 completed passes44 completed forward passes13 passes completed under pressure9 progressive passes9 passes into final third7 shot-creating actions1 completed through ballAnother fine passing display. 🇭🇷 pic.twitter.com/d13yoULHP8— ExpectedChelsea (@ExpectedChelsea) March 9, 2021Playing under Maurizio Sarri, Kovacic struggled to hit his stride in his first 12 months with the Premier League club, as he was heavily rotated with England international Ross Barkley.However, with the appointment of Frank Lampard, Kovacic became a key man for the Blues, as he dazzled at the centre of the park.Kovacic was named Chelsea’s player of the season last year, after a stellar campaign where he made 47 appearances across all competitions.#8 Timo Werner | £47.6m (2020)Chelsea v Everton - Premier LeagueAfter dazzling in the Bundesliga with RB Leipzig, Timo Werner became the hottest striker on the market, with a plethora of clubs across Europe keen on his signature.The German international hit 28 goals in 34 appearances in the Bundesliga, including creating eight assists for his teammates, in his last season at Leipzig.Interest from the Premier League was particularly high, with Liverpool heavily touted to snap up the hit-man. However, the Reds stalled in making an offer for the German international due to the financial implications brought about by the COVID-19 pandemic.Most penalties won in the Premier League this season: 4️⃣ 🇫🇷 Anthony Martial 4️⃣ 🇩🇪 Timo Werner 3️⃣ 🏴󠁧󠁢󠁥󠁮󠁧󠁿 Patrick Bamford 3️⃣ 🏴󠁧󠁢󠁥󠁮󠁧󠁿 Jamie Vardy | #MUFC | #CFC | #LUFC | #LCFC | pic.twitter.com/mZTehyPGgo— EPL Statman (@EPLStatman) March 10, 2021Chelsea, who were going through a squad overhaul following their transfer ban, acted swiftly and triggered his £47.6m release clause as Frank Lampard signed the German to spearhead his attack.While Timo Werner has struggled to find his scoring boots with Chelsea, he has occasionally shown flashes of brilliance and the quality he possesses in attack.His electrifying pace makes him a constant threat to opposition defences, as he often drifts wide to the left and finds pockets of space to exploit or lead counters by simply running at the defence.#7 Ben Chilwell | £50m (2020)Chelsea vs Wolverhampton Wanderers - Premier LeagueFollowing a poor defensive outing in the 2019-20 campaign, where they shipped a whopping 54 goals in the Premier League, Chelsea opted to bolster their squad in all departments.The left-back duo of Marcos Alonso and Emerson Palmeri had grossly underperformed, prompting Frank Lampard to dive into the market for a new defender.While the Blues were linked with a plethora of full-backs, they completed a move for England international Ben Chilwell from Leicester City for £50m.Chilwell instantly became a key man for Chelsea, and he enjoyed a stellar start to life at the club, bagging two goals in his first five league games.However, following the dismissal of Frank Lampard and the appointment of former PSG boss Thomas Tuchel, Chilwell has struggled to pin down a regular role with the Blues.