Cost effective flooring under indoor trainer

OK. Here's the whole briefing from a gym part-owner. Probably goes wider than you need but hopefully useful.

You don't say what your trainer setup is - do you mean a roller-trainer, or a rowing machine, weights, or for doing pressups and situps on?

Is it supposed to cushion your body, let you drop things, or provide a quiet base for a machine? How wobbly can it be?

Having setup a couple of crossfit gyms in the last decade, there are several options. There are similar ones (eg a thinner version of 3) that may meet your needs. Getting a good understanding of your needs is the most important thing.

1 - You can get lightweight coloured foam mats that go together like a jigsaw. Gyms short of money may use these for eg childrens' areas or warm up areas where members do stretches and so on. These are dirt cheap and appear from time to time in Aldi Special Buys and so on. Often they will be 2ft x 2ft. These are soft but yielding and around 6-10mm thick.

Imo these are not suitable for a proper exercise floor and are not a firm base, through OK for a warm up stretching aream, and perhaps Yoga (but your sit bones may feel bruised). But they are cheap enough that you can try them then throw them away.

Example: https://www.amazon.co.uk/Grendle-Black-30-5-Interlocking-Foam/dp/B08K91QPZM

2 - You can get stuff that comes on rolls that is perhaps 3-6mm thick, often with a pattern of circles on it. Or sometimes  finish like a hairy rubberised material.

We use these for walking and running around areas, and walkways between areas of 3, glued down. We then put option 3 for areas where people exercise on the floor or eg boxes, or where weights may be dropped.

These are not suitable for doing sit down or lie down exercises on, but are probably OK for eg running up and down, or doing bear crawls etc.

Example: https://www.ebay.co.uk/itm/125733699969

3 - Then there are thick rubber matts that are ~12-20mm thick, and come in 6ft x 4ft usually or 1m x 1m. Each one weighs about 25-35kg, and can take weights being dropped.

We use these single or double thickness depending. They are suitable for ground exercise such as situps, and for weights. They can be a good base for a machine.

You want this stuff to land on from height or for working upside down - eg pull up frame or hand stand press ups.

We were paying £40-50 in a good deal on 18-20mm 6ft x 4ft ones of these wholesale buying enough for a whole gym back in 2018. Still going strong in a busy gym. Thinner ones may be OK for machines.

Examples:
https://www.ebay.co.uk/itm/164495346317
https://rubber-gym-flooring.com/product/premium-gym-flooring-rubber-20mm...

4 - How to buy.
a) The important physical things are degree of softness, density and thickness. Don't let anyone sell you 1 as 2 or 2 as 3, or a thinner version as the real thing. That's a false economy. Better to save up for the right stuff than risk injury.
b) Suggest you take up the free month membership at your local gym (suggest a Crossfit or similar gym as they have cycling sort of kit and do floor work), to learn about surfaces and work out what you need.
c) For 2 and 3 look for suppliers to gyms close to you to collect, or free delivery online with a 14-day return right. 2 and 3 are heavy and carriage can overwhelm cheap price. Amazon Prime free delivery may be useful.
d) When you know what you want have a word with a gym where you or family are a member and ask if they have anything in their storeroom, and can you have some if you make a suitaby generous donation to their gym charity.
e) Choose your sizes carefully; there are some exercises you do not want to do across joins if you are not gluing it down (which needs VERY good glue - eg Evostik Seriously Strong Stuff.). 
f) 3 above is quite like horse stall matting. So visit your local equestrian / agricultural supplier for a look-see.

All the best.

I have a £22 mat, like this:
https://www.fitness-superstore.co.uk/body-power-48-x-36-floor-protector-...

It's different to a yoga type mat as it's very heavy and doesn't move around. Reduces sound and vibration a bit as well as protecting the floor.

They have a range of sizes...
https://www.fitness-superstore.co.uk/fitness-equipment/fitness-accessori...

B&Q do interlocking foam mats. Can't remember the exact price but I think it was around £10 for four mats.

I've found a doubled over yoga mat offers enough vibration absorption; if you keep an eye out as you ride around loads of people seem to leave them outside their houses for any takers, particularly this time of year – possibly indicative of new year resolutions falling by the wayside!

You can get rubber feet and mats for washing machines, that should work.