Depends hugely on your volume and also on some buying choices when it comes to cycling. Swimming costs are pretty fixed regardless of volume (pool membership is high, swim suits and goggles are cheap and last a long time). Cycling and running costs rise the more you do.

Personally, when I'm swimming regularly enough to need pool membership, it's my most expensive sport. I'm in London and access to a decent swimming pool costs more like £1000/year. That gives access to a gym and classes as well, but I don't do classes and can do enough strength and indoor training at home that I wouldn't bother with membership unless I needed the pool.

Running is the cheapest, I don't really track how many miles I get out of a pair of shoes but I certainly ignore the advice of the manufacturer and don't think I've ever got through more than 2 pairs in a year. Even if I buy some running kit as well then that's still less than half the cost of pool membership, and some of my running kit is 10 years old and still going strong.

Cycling depends whether I do the actual cost, or the "wife cost" which factors in that I commute to work by bike which saves me literally thousands of pounds a year and by my spousal accounting system makes cycling a profit centre and not actually a cost at all! Even if I just look at the expenditure then I typically spend less on cycling than pool membership would cost me. I have a mountain of kit but some of it is >10 years old and I just add a few choice bits each year when there's a sale on. I do all my own bike maintenance and bought most of the tools I need years ago. Bikes typically last me over 10 years, so annual cost isn't that much, and I do most of my training on fairly cheap and long-lasting wheels and tires.

My personal experiences on a scale of 1 to 10 with 10 being the most expensive:

Running: 3
Biking: 6
Swimming: 6

Loss of time driving to the pool then bumps swimming to a 9.

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"If it costs you 30 minutes at Maryland so what" -dwreal

For me, it is swimming is most definitely the cheapest. My employer has a gorgeous 6x25y pool. There's rarely lane sharing, no commute (I'm already in the office) and no cost.

You'd better be a FFOP swimmer, then!

Gym membership is $50 U.S. per month, so $600 a year. The only reason I have that membership is the pool.
I only go through 2-3 pairs of shoes per year so even if I assume $100 a pair, swimming's still more expensive.

Funny. Quite the opposite. Check the AG Nationals Olympic M55 results. I was LAST out of the water :-). Thank goodness I can run reasonably well.

I wear out one pair of shoes per years on ~600 miles/yr. I usually pay about $80 for them. Brooks PureFlow and/or Asics Gel Hyperspeed have been my trainers for the last 5 years. They are not rated to 600 miles but some of my winter mileage are done on the treadmill where shoes last forever.

I wear out a $30 swimsuit about every two years and a pair of goggles per year.

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Madison photographer Timothy Hughes | Instagram

Swimming for sure cheapest for me. 2 free public pools nearby - one 2 miles away, the other 4 miles away. Both with long course during lunchtime, and short course early evenings.

So, it's just jammers and goggles. Even counting the somewhat frivolous (for me) Garmin Swim, center snorkel and wheelbarrow tube (Guppy training!) last year, it's still the cheapest by far.

Where do you live that has such riches in public pools?!

Honolulu. :) The only drag is that I'm much farther from decent open water swimming (distance + traffic). I know, life is rough ... ;)

For me definitely the Swimming, but I am lucky in that the college in my town lets the Tri group we have use the pool for free 3x a week in the evenings, for most of the year, a few winter months we do not have the pool in the evening due to the College letting the High School team use the pool. Most of us do pay the yearly fee though of $75, that way if we want to go in the morning or on Sunday we can. I then go through 2 pairs of goggles usually. So between $100-115 a year.