I purchased this expansion in the steam winter sales. Now treating this game as 100% arcade and using the gamepad instead of a wheel (although still using hardcore mode), the new chase mode is pretty fun, and the core gameplay mechanics are well executed. I find it to be a proper pick-up and play alternative to Rocket League for killing 30-60 minutes now.
Nevertheless, and many steam reviews echo this, I just can't see the value for money. There might be quite a number of police vehicles, but with only one cop shop and about 7 slots with high prices, it could take ages before you get a chance to buy the one you want. The extra story missions are over very quickly and do not offer anything that unique over the original (which already had harder chase + takedown missions). It is good that all players can 'be chased' without the DLC, but if you really want to chase others, there's nothing to stop you spending $0 and trying to stop the player with a non-police car - you just wouldn't have any "abilities" that effect the escaping player.
The 'crates' progress appears to be reset each day. If you're good, go straight for the Platinum ones - the distance is a bit longer but I don't find it that much harder to escape. It's a good way to quickly level up a car as well.
TL;DR
If you don't mind the handling, try out the 'being chased' side for free. If you like the mode and really want to try chasing others in a proper police car, get the DLC when you think the price is reasonable. I'd suggest ~$15 AUD as reasonable - you're probably never going to get to use all the new cars. You still require a good $400,000 in game credits to buy the various spec police cars (Dirt, Performance, Raid, Circuit) to complete the approx. 10 missions. Once they are done you'll probably only ever use your fastest Circuit and Raid police vehicles.