Not enough information has been provided for an accurate answer. You wrote that you want to broadcastify a FREQUENCY from a P25 trunking system--makes no sense to me...perhaps you meant a specific TALKGROUP. You didn't state what system--if you are referring to the Morris County NJ system (based on your listed location), it's a simulcast system and an older/cheap scanner may not give you desirable results. If you actually wish to broadcastify a specific frequency, then anything capable of decoding the signal MAY work...but to follow a TALKGROUP, no. The cheapest option would be a Motorola XTS5000 from E-Bay for under $100 with a battery eliminator for around $15. You need someone that knows what they are doing to program it correctly for Non-Affiliated scan or you will end up transmitting...thereby not passing go, not collecting $200, and go directly to jail (rolling doubles won't get you out). You asked for cheapest, not easiest. Barring that, 436, 536, SDS100, SDS200 are your choices from Uniden...none of which are cheap. Read up on similcast systems...some have luck with older scanners--most don't. I recently sold my 436 and 536 and bought the SDS100 & 200 because of simulcast.
If you are asking about the Statewide system, you need to find out if the desired talkgroup is phase 1 or phase 2. Motorola XTS5000 will only work for phase 1.