Low-profile subwoofers are expensive because it gives you the best of two worlds -- both sonically and dimensionally. As we know, most subwoofers are rather bulky. Trying to compress the same subwoofer while not compromising the sound it gives off demands a premium
IMO, I'd rather go for an actual subwoofer with an amplifier. Shouldn't take too much space as long as you don't buy two of them. It would probably sit on one part of your trunk. This way, it would give you the flexibility of further upgrading your sound system in the future.
Some amplified subs are placed under the driver's seat. Of course the bass doesn't reach as low as what a normal subwoofer is like, but since it's just sitting under your butt, you'll still feel the effect/vibration of the bass. And most casual users just want to hear that "thump"
Most sound enthusiasts just replace the front speakers. Replacing the back is kinda waste of money unless you spend more time sitting at the back. Since most of us are drivers, we opt to replace the front speakers and just retain the back speakers as stock.
If you really prioritize cargo space, then an amplified subwoofer is the most cost-effective way on getting those low-frequency sounds on a budget. If you're a driver, I wouldn't recommend installing JBL coaxs at the back. I'd rather use the cash for a better sounding front speakers from Morel or something. As long as it's around 5-6k, your HU should be sufficient to drive them. Just check the HU's RMS output and the speaker's RMS output just in case.