Should I build plinths or screw spikes into cabinet?

guitared, You should go under your house, find the area to support with bottle jacks and 4x4's. (or equivalent), After this, (if needed),you may look at look at other options above.

(All of this will keep you under your $100.00 target... I'll make a bet that it will be the ticket...just requires some effort.)

Be careful if there made of press board. Threads are a good idea (already mentioned). There are several good options. If you send me a PM I will show you something I came up with for ye old English towers. It widened stance and has them firmly planted. It made a sound improvement.

One option thats OK. If your going to go right into the cabs cut some 1/4" pieces to bond to the bottom, and uses spikes with wood screw type threads on opposing ends. IMO this is more durable than some of the other options. 

My post was out of what I feel to be the need to address the main issue first: your suspended wooden floors. (I have a similar issue) This should be done first and addressing this basic issue will be beneficial for every aspect of your listening pleasure and system upgrades from now on.
Good luck!

The main reason I like (spikes), it allows me to fine tune speaker placement which I find to be critical. If you don't feel a need to drain vibrations away, the Vibrapods may be one solution. Some may have monitors that allow this adjustment to be made between the speaker and the stand.

I've found that with suspended wooden floors, there will be (out of level) floor issues that require a more thoughtful approach that may include having several methods of adjustability.

Thanks again to everyone for the helpful responses.  

I just put two hardware store sourced jack-posts on top of cinder blocks in my crawlspace directly under the speakers, and wow what an improvement!  The low end has evened out through the lower frequency range where before there were certain bass notes that rang much louder than others.  I believe the level of detail in the mid and upper ranges is up significantly as well.  I read something recently about working to allow your current gear to sound as it was intended before investing money in new equipment.  It has been a fun learning process figuring out cheap DIY methods to get significant improvements out of the equipment I have been listening to for a few years now.  If my system wasn't in a family living space I would move on to acoustic treatments next, but I'll have to stick with creative use of pillows and blankets until we move to somewhere I can get a more dedicated listening area.  I already got shut down for the more "attractive" items by my better half haha.