I spent 8 years in the avionics industry. There's no free lunch and aircraft are expensive, there is literally no work around for this. Ads-b is here to stay so expect to be compliant by 2020. Transponders, audio panels, radios , GPS, and everything else adds up very very quick. If you don't have the cash, don't buy it.
I should elaborate. The faa mandate for ads-b requires a mode c transponder, an ads-b tranciever, and a waas GPS. If this airplane is not already outfitted expect to pay around 28k for minimum equipment garmin flavor.