Not advertising here but just a few examples:-
"Garmin has introduced a value-priced Mode-S transponder, the GTX 330. At $4,995, the GTX 330, Garmin believes, will be a cost-effective option."
"Bendix King KT70: Similar to KT-71 but DOES include Mode S (Select) capability "
GTX 330 Panel mount, Mode S datalink transponder (black face) $4995
GTX 330 Panel mount, Mode S datalink transponder (gray face) $5237
GTX 330D Panel mount, Mode S datalink transponder with diversity (black face) $9995
GTX 330D Panel mount, Mode S datalink transponder with diversity (gray face) $10487
KT-70 Solid State Digital Panel Mounted Mode S Transponder OHC
*includes installation kit upon request $2750 $995
KT-70 Solid State Digital Panel Mounted Mode S Transponder NEW $5504 $---
KT-70+ Silver Crown Plus Solid State Mode S Transponder NEW $5504 $---
[Not sure what enhanced is....

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EA