Right - you are talking about the screw on valve caps and yes they are MUCH cheaper than the valves that send a signal that you insert into your wheels as per the Tire Insight kit that I was offering you.
I have tried 4 different styles of the valve caps and all kits proved, lets say, 'less than satisfactory'. Partialy due to inaccurate pressure reading but more significantly inadequate communications meaning that I had to often reset the caps to regain contact.
Perhaps the latest versions are better but if you wish to try one type that I still have PM me and I can post a new set to you for the cost of the postage. They should should be able to communicate with your head as they should be ok with an Android phone app . However they have their own display unit that you can suction fit to the dash or screen and as long as it can see the sun will keep itself charged by way of the integrated solar panel. They have been tried on my car but the kit is in effect new.
It wasn't myself who was looking for a set, BTW.
Personally I have had TPMS systems that use internal sensors (replacing valves) and external sensors (replacing dust caps) and I have found zero performance issues between the two.
Of the two styles, I actually prefer the external ones, but I know many who think they are problematical. I have not seen that problem and are maybe 50/50 split in terms of the various TPMS setups I had over the last 18 or so years between internal and external versions.
In terms of cost, the kits with Internal sensors are not any more expensive than ones the external sensors.
Where Internal sensors ARE more expensive is when you are stuck with replacing a failed OEM sensor as the dealers charge rediculous prices for them, not reflecting the true cost of them whatsoever.
Merl, if you are looking for an TPMS system that will interface with an Android Head Unit, here are a couple of options to look at:
System using External Sensors -
https://amzn.to/4aw0Ahn - £41 (This is the model I use with my Android head Unit)
System using Internal Sensors -
https://amzn.to/3RAyZDc - £37 (same Android app and USB connection box, just using the internal sensors instead)