Lionel Messi will be a free agent when his Barcelona contract expires this summer. 

PSG and Manchester City are two of the clubs hoping to take advantage of what will be a historic bargain (aside from his astronomical wages, of course). 

Compatriot Sergio Aguero is in a similar situation as the clock ticks down on his current Manchester City deal. 

Indeed, we're in an unusual situation whereby some of the world's best players are going to be on the market and they won't cost a penny to sign. 

In the cases of Messi and Aguero, the one thing that may put suitors off them is their age. 

Both are now in their 30s, but they are accompanied by a long list of contemporaries who are still valued highly even as they enter their twilight years. 

With a little help from FourFourTwo, we've decided to look at the 25 players in world football right now with the biggest price tags after their 30th birthday - and there are a few surprises. 

=24. Kieran Trippier - £18m (Atletico Madrid)

=24. Felipe - £18m (Atletico Madrid)

=18. Karim Benzema - £22m (Real Madrid)

=18. Marco Reus - £22m (Borussia Dortmund)

=18. David de Gea - £22m (Manchester United)

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=18. Jordi Alba - £22m (Barcelona)

=18. Stefan Savic - £22m (Atletico Madrid)

=18. Allan - £22m (Everton)

=16. Angel Di Maria - £25m (PSG)

=16. Jordan Henderson - £25m (Liverpool)

=14. Sergio Aguero - £31m (Manchester City)

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=14. Wojciech Szczesny - £31m (Juventus)

13. Thomas Müller £32m (Bayern Munich)

=7. Miralem Pjanic - £36m (Barcelona)

=7. Georginio Wijnaldum - £36m (Liverpool)

=7. Ilkay Gundogan - £36m (Manchester City)

=7. Pierre-Emerick Aubameyang - £36m (Arsenal)

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=7. Kyle Walker - £36m (Manchester City)

=7. Wissam Ben Yedder - £36m (Monaco)

6. Ciro Immobile - £40m (Lazio)

=4. Toni Kroos - £45m (Real Madrid)

=4. Eden Hazard - £45m (Real Madrid)

=2. Cristiano Ronaldo - £54m (Juventus)

=2. Robert Lewandowski - £54m (Bayern Munich)

1. Lionel Messi - £72m (Barcelona)

Messi is still the most valuable over-30, ahead of Cristiano Ronaldo largely because he's two years younger. 

Ronaldo and Robert Lewandowski are tied in second spot, though neither forward is expected to be on the move any time soon. 

For others, their price tags are still impressive, but mark a sharp decline. Real Madrid signed Eden Hazard in a deal that could rise to £150m, but less than two years on and he's worth a fraction of that now due to his injury struggles and difficult start at the Bernabeu. 

Many clubs will not be getting the benefit of boasting some of the most valuable veterans around. Aside from Messi, Georginio Wijnaldum is also set to leave Liverpool on a free transfer this summer and they are not in a position in central midfield to cash in on the Dutchman now.