Tunnel time is not cheap. For casual flyers there is no real way to make it be cheap, short of selling your soul to a tunnel company for a position as an instructor. However, there are a few things to learn about the process of procuring time that can help make every minute as useful as possible.
The important bit of information here is the more time you buy the less expensive it is per minute.
For example - if you but 10 minutes of flying you will be paying a standard rate, but if you buy an hour you will qualify for a slightly cheaper category and save a small percentage on each minute. If you buy five hours you might qualify for the next cheapest level and save a little more. To qualify for the lowest rate that the facility offers you may have to commit to something like 20 hours of time. This is a lot of money to stump up for tunnel flying but if you are committed to getting good and have to cash to invest in it early then the savings start to make sense.
Generally speaking there are a couple of ways to buy time - either from a tunnel or from a coach.
Coaches make their money by purchasing time at the cheapest per-minute rate then selling it on to their students at a higher price and banking the difference as the fee for their services.
As would make sense for an industry where different standards and levels of experience are available - coaching fees are not all the same. A multiple world champion with many years of experience might cost you more than a new instructor with a year of working at the tunnel under their belt. However, a general rule is:
If you are buying smaller amounts of time the difference in price of buying form a tunnel or from a coach is negligible.
The advantage of buying from a coach:
You get to learn stuff in a structured and efficient manner and you do not rely on the uncertain system of being coached by tunnel instructors.
The advantage of buying from a tunnel:
You may be at the stage in your flying where you can choose wether you need coaching or not. If you just want to zoom around practice without having to talk to anybody then you can. If you do want coaching for a session that you have already booked you can pay someone separately as a separate arrangement. If you want to invite your friends for some group play you can.
Important: If you are relying on the tunnels instructors to teach you, remember that they learn on the job and might not be qualified to spot what you want to learn. Tunnel facilities often have a system with which to request an instructor of a high enough level to cover what you need, so don’t forget to ask (A good coach understands this system and will make the appropriate arrangements automatically). Remember that the more advanced you get the greater the chance that a tunnel instructor will not be able to teach you.
For the majority of casual flyers it makes the most sense to buy time through a coach. This is because the best way to learn at the tunnel is from and experienced flyer that can effectively and efficiently communicate ideas, demonstrate techniques and provide a quality one-on-one service. The pool of instructors at your local tube may well be good coaches, but are often simply too busy to offer sufficient depth - and if the money you are is not really any different then the choice is an easy one.
Look out for loopholes: If the tunnel is running a special promotion you might be a be able to take advantage of it. Buy-One-Get-One-Free on introductory flight packages you say? How many can you buy? Likely nobody cares that this is not proper pro-flyer time - certainly not the instructors. They would probably much prefer to teach you something for a session than process a family of newbies.
Events: Tunnels might run special events for group flying - tunnel scrambles, night parties etc. Organised group events can be bags of fun and a great way to find flyers of a similar level to practice with.
Last minute rates: If a tunnel has a particularly quiet period or a big group cancels at the last minute they might offer the empty time at a heavily discounted rate. This is often a regulars-only type thing - make nice with the right people and be sure the tunnel has your contact details.
Locations: There are a lot of tunnels now, and a lot more on the way. Some places where you can train are cheap to fly to and cheap to live in. Depending on how much time you want to do it can cost less to go abroad than visit your closest tunnel. Plus you get to see somewhere else and maybe learn some stuff.