Denver has such a requirement as an amendment to the IFC (gotta love how Denver officially amends the code and publishes policies).
SECTION 915
RADIO ENHANCEMENT SYSTEM
915.1 Radio Enhancement System for underground buildings. All underground buildings shall comply with
Department of Safety Radio Enhancement Systems (RES) requirements as defined in IFCA Section 907.2.12.3.3.
915.2 Radio Enhancement System for buildings exceeding 50,000 square feet of building area. All buildings
exceeding 50,000 total square feet of floor area shall comply with Department of Safety Radio Enhancement Systems
(RES) requirements as defined in IFCA Section 907.2.12.3.3.
The Department of Safety radio frequency range—800 MHz to 870 MHz—shall operate effectively throughout the
structure at a radio control channel signal level of no less than -100 dBm at 90% or more of the locations measured within
each grid area. Equivalently, the service area reliability shall be 90% or greater on each floor of the structure and parking
levels.
Upon completion of major construction and prior to occupancy of the building, radio acceptance testing by Denver Fire
Department Technicians or technicians with appropriate knowledge and training who are licensed by the Denver Fire
Department shall be performed.
Exception:
Where an RES is not installed at time of construction, an empty two-inch (2”) vertical metallic raceway or conduit
shall be provided near each required stairway enclosure and in each elevator lobby. Where the radio signal
coverage does not comply with provisions of Sections 915.2 and 907.2.12.3.3, the 2” empty conduits shall be used
for installation of RES. Installation of RES shall be completed prior to issuance of the Certificate of Occupancy.
This is being enhanced for the next go-around of code adoption/amendment, and may have a retroactive requirement.
Sorry, but I don't have a good budget number for you. I do know that it's a huge "it depends". Many times, all that is needed is a passive antenna system such as "Leaky Coax", not necessarily a powered repeater.