Assuming you already have an installed units with all ductwork and so forth, small square footage furnace and AC can generally run anywhere form $2000-$5000 for each unit, or about $4000-10000 for a combined unit, depending on ease of installation, etc. This assumes your existing plumbing, electrical, ductwork, etc are directed suitable, so it is merely a matter of cutting in a new unit and hooking it up. The lowest price would be for a minimally efficient unit, the upper number for highest efficiency.
If you are changing the type of heat (baseboard to forced air or electric to gas, for instance) or installing AC from scratch, then running power, ductwork, etc can up to double that number.
If you live near the coast (and depending on how hot you will let the condo get before cooling kicks in) the cost savings of a high efficiency unit might not pay for itself. Of course, if you are in the San Gabriel Mountains and really need winter heat, or in the Mohave desert or inland valley area and really need serious air conditioning, then a high efficiency unit (which can cost twice as much) might pay for itself in 5-10 years.
Your utility company (companies if gas heat with electric AC) can help with guides on how much your savings might be with different efficiencies, and may have an energy efficiency rebate program for efficient units. Also, the California Energy Commission has information and online calculators for that, as well as rebate programs for energy efficiency. There is also a federal energy efficiency rebate program (though it is generally limited to a tax credit equal to 10% of the cost of energy efficient retrofits, unless it is considered an "alternative energy" system). Pay attnetion to these energy rebate programs - I have a neighbor who, between state and federal program, had the entire cost of his insulation upgrade and furnace/hot wate heater system replacement covered in toto, except for few hundred $ for the initial energy audit. About $8000 in work for about $300, with about $100/month reduction in gas bills.
As always with condo units, check with the condo manager and the bylaws to find out what the rules are - some condos require matching units or installation by a certain contractor for ease of maintenance, and in many condos the heating and AC are actually the responsibility of the condo association or the bylaws require that the association handle the installation but you pay the cost, etc. Also, major work of this type usually has to be coordinated with the condo manager, and there may be time of day limitations on work hours, etc.
And of course make sure your HVAC contractor, if you end up choosing him, is licensed, bonded, insured, and well recommended, and whether any repairs to walls, floors, ceiling etc necessitated by replacing the system are or are not covered in his bid (he would subcontract this out, or more likely tell you that you need a general contractor to supervize plumber, electrician, plasterer, painter, flooring contractor, etc).