There are two styles of trainer that I'm familiar with: Magnetic (or mag) and fluid.

Mag trainers are the cheapest types and mimic outdoor riding the least. They can certainly give you a good workout, but your MPH per HR on a mag trainer will make you think you're a super racer. So you can't really rely on your mileage as a guide for your cycling chops.

Fluid trainers do a better job mimicking outdoor riding. They're usually more expensive than mag trainers, though not necessarily by a whole awful lot.

Many trainers offer intensity level adjusters, to mimic hills/flats. Some, you have to get off the bike to adjust, others have a switch connected to a wire that reaches to the front of your bike. So, you can pull this gear switch thingy and automatically be "going uphill". Personally? I LOVE this feature. It breaks up the monotony of a trainer ride to be able to pull a switch and suddenly go 2 HR zones higher. If you're into hill riding, it's almost a must (in my humble opinion).

I've done spin classes but to be blunt, I much prefer trainer riding, as I do it on my own bike. Yes trainering is still quite different than road riding. But it still has a truer "outdoor" feel than spinning, since you're actually on your own bike. Maybe you can find a place that allows trainers? There's a local tri shop near me that does computrainer classes almost every day. They've got a computrainer studio set up with 15 trainers.

Performance Bike Shop has a wide selection of trainers, at affordable prices, if you're into online shopping. My personal recommendation is find an adjustable intensity fluid trainer. Whatever you go with, I hope you enjoy it.

I hope this helps.

I also have the magnetic type (TACX brand) and I`ve had it for almost 10 years without any issues. Mine has the adjuster and the way it`s setup now it`s pretty close to the resistance I experience outdoors based on my HR and speed.

Like someone said, look for something better that`s used than getting something new that`s cheap.

USED! They're so easy to find! We got a used mag trainer for $29 (had it for 3 years) and a used fluid trainer for $50 (we've had it for 2 years and love it). They've both been great pieces of equipment. Our brand spanking new mag trainer? It makes a great bike stand, but not much of a trainer.

Yes. Used.

Trijoe and Jacksonpt are right on, you can find a USED trainer (I have a cycleops wind trainer - noisy) on craigslist as well. Look around. Additionally continue to attend spin classes - they work you out differently which also helps. BUt get out on the road is always the best.
Look into a couple of additional products as well. Shoes - if you dont have cycling shoes with clips look into them. It will save your legs for the run. You are able to pull up while you are spinning and use your quads in addition to your hammys. Also - aerobars. You canfind these relatively cheap. Practice with these as you change your position on the bike. The aero position will help you go faster and save energy for the run. I got mine for $35 from Performance Cycling.
Its a grat sport and fun time but can get pricey the more you get into it. Start out small and build up the more you start to like it.
Finally - do your best. I suck at the swim so when I get on the bike I find the next person in front of me and chase them down and then the next, etc.
HAVE FUN.