Brownells has a Colt M4 barrel assembly with front sight tower and barrel nut for $320:
https://www.brownells.com/aspx/search/productdetail.aspx?sid=250617&pid=137364
[EDIT]
Here are lower cost alternatives (thanks Outrider). Yes they are out of stock, but you can get on a notification list:
https://www.gandrtactical.com/Colt-145-M4-SOCOM-Barrel-w-FSB_p_332.html
https://www.gandrtactical.com/Colt-145-M4-Barrel-w-FSB_p_328.html
https://www.brownells.com/aspx/search/productdetail.aspx?sid=250617&pid=137364
[EDIT]
Here are lower cost alternatives (thanks Outrider). Yes they are out of stock, but you can get on a notification list:
https://www.gandrtactical.com/Colt-145-M4-SOCOM-Barrel-w-FSB_p_332.html
https://www.gandrtactical.com/Colt-145-M4-Barrel-w-FSB_p_328.html
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