The cost will depend much on what standard of installation you want. The standard you need depends on proximity of transmitter and or the range you want.
So if you only want TV in the marina close to a transmitter thena very simple set top antenna in the cabin may suffice. Hence if you have a TV that runs on 12v from the battery or you have 240v shore power then really harly any installation cost. Assumining you can mount the TV.
You may want or need to run the TV on a regulator off the 12v battery or you may want to run the TV off a 240vAC inverter. More cost and installation.
The antenna then will dictate for free to air what sort of range or performance you get. An antenna up as high as possible means often up the mast. hence cables up the mast.
An antenna can be a omni directional antenna which means that while sailing or regardless or boat orientation on an anchor you get a steady signal. These usually have a built in amplifier to amke up for lack of gain or signal strength from the omni directional function.
The other type of antenna will be directional a bit like antenna at home. This must be pointed at the transmitter. Hence boat must be constantly facing one direction. (in a marina)
So all this before you think about satelite TV or internet TV. So what do you have in mind? olewill
I have a Bantem 335 onmi aerial at the masthead, feeding a Cello LED TV. This is a 12 volt TV and amazingly enough made in the UK! This works very well, coverage in west of Scotland depends very much on location, and after moving some distance it can be necessary to retune the TV, which is a simple process. I have been advised that 12 volt TV sets can be sensitive to power supply voltage. Modern battery chargers and intelligent alternator regulators can output at fairly high voltage especially when in de-sulphate mode. For this reason I have recently invested is a voltage regulator at about £25. The TV was approx. £180 and the aerial around £80.
Angus
Our boat had a Glomex fitted when we bought it but it's never picked up a signal in our marina, it appears that the nearest transmitter is vertically polarized and I have had mixed advice that some Glomex aerials only receive horizontally polarized signals. The mast is currently down for maintenance work - I bought a new Glomex Talitha which receives both vertical and horizontal signals at a cost of £90 from Force 4. It comes with 20 meters of coax and a 12 volt booster so the only additional cost is the masthead bracket and the television unit itself. I believe the masthead bracket is around £50 but you may not require this for gantry mounting.
I previously had an off the shelf household TV which had a transformer converting 240v to 12v. This had been run straight off the 12volt domestic boat system and subsequently caused problems when the shorepower or engine was running (giving around 14v). A voltage regulator or inverter would have been a better idea. To keep things simple I then changed it for an Avtex unit which can work off mains or 12volt (or whatever your shorepower or alternator provides) and this has worked well for a number of years.