Glasgow to Lochranza (Arran) - cheapest way to buy return tickets?

tl;dr: all told the single (one-way) journey would be around the ballpark of £20, and you don't need to buy anything in advance. The price would be more or less double this for a return (ie round trip) journey.

I'm assuming here you're going from Glasgow Airport, the non-rail-connected one by Paisley. If you are instead going from Glasgow Prestwick Airport the instructions will be different as this one is rail connected.

The bus from the airport is probably best to pay for separately. McGill run a 757 bus from Glasgow Airport to Paisley Central Road right outside Paisley Gilmour Street station, which is already partway along the line towards Ardrossan Harbour, so unless you want to spend some time in central Glasgow this will probably be your most efficient option. You buy your bus ticket from the driver; I don't know if they accept card here so it might be worth ensuring you have some cash (and preferably change for a bus). I don't know the fare either; bus fares are annoyingly hard to work out for some operators, but it'll probably be in the realm of £2-£4 I imagine for a single ticket.*

The rail and ferry fare can be combined as part of a scheme called Rail and sail. As that page explains, there's no need to book these in advance; you get the same prices if you just buy the ticket at the station. You will want to buy your ticket to Brodick, which is the name of the ferry terminal on Arran. There are ticket machines at Paisley Gilmour Street station you can buy these from, or speak to a human at the ticket office which has long opening hours. The fare from Paisley to Brodick right now is £11.20 for a single (as far as return (round trip) tickets go, only day returns are available on this route, which are only valid for one day). If you will be coming back you probably therefore want to purchase both tickets, and I'd recommend you do that at Paisley station as I'm not sure it's possible to buy them in Arran. You can ask the ticket office staff for a single going back on some other day, or the ticket machines might be able to do it (I don't recall what functionality is available on ScotRail's ticket machines). If you're willing to pay just slightly more for a bit less hassle, or if you intend to visit Glasgow itself, there is a return ticket valid for 1 month available from Glasgow stations to Brodick for £22.90 (this is valid for starting or ending your journey at Paisley Gilmour Street since the latter is on the way to Ardrossan Harbour and there are no break of journey restrictions on this fare).

Trains to Ardrossan Harbour from Paisley are hourly; a slightly overcomplicated timetable can be found on the ScotRail website or you can simply use the journey planner on their homepage. Connecting ferries including which train arrivals they connect with are detailed on the CalMac website. Your ticket will not tie you to any particular train or ferry, as long as the date you're travelling matches the date on the ticket you should be fine!

On Arran you will be catching a Stagecoach 324 bus. Again frustratingly their website's fare finder doesn't appear to work for this route; but again I don't imagine the fare will be more than a few quid. Again you would buy it from the driver; Stagecoach I would expect would have working readers for contactless credit or debit cards.

* Those who are familiar with combined rail and bus fares might now be screaming at your computer screen, "but what about PlusBus?!". Well, unfortunately, the PlusBus website says it's not valid to Glasgow Airport. There ARE special fares for travelling from the airport to Ardrossan but these don't exist in conjunction with Rail and Sail. Since it's so hard to find fares for this sort of thing I can't say which would be cheaper, but my instinct would be paying for the bus separately and getting Rail and Sail would be cheaper. In any case it probably wouldn't be possible to actually buy this special fare from the bus driver! If anyone knows otherwise do let me know!