What is the cheapest form of waterproofing?

use any curing compound on the surface of fresh concrete and after initial setting get started start floating blade commonly saying hilicopter over top to provide smooth finish this will help to fix the top pores which are the main factor alow water to seep inside concrete easly.

Is it possible to water proof a first floor flat in an already built 20 year old

four storied apartment building? Access is to only the one flat belonging to us. Seepage from above spoils the paint and drips from bathroom walls and ceiling.

I was thinking whether small holes if drilled in the seeping walls could act as drain points and carry out the seep water, which could be piped away or let go to the floor drain, etc.

Alternatively is it possible to pressure inject some epoxy like compound to fill up the pores and block seepage paths, and will it adversely affect the strength? How much pressure is safe to apply in an injected liquid?

I agree that commonsense is the best thing. Modern day are unsuitable for our weather conditions. We have heavy rainfalls and severe summer. The best and most economical water proofing  of roof slab shall be to have a good slope, by water flows off easily, and preferably, in olden days, people used to do simultaneous finishing of the roof,when the slab's initial settlement has started, so that the Mason can walk on that and apply 1:1 CM with his trowel like power floating and provide 6mm groovings of size 2'X2' sq., throughout the slab. Secondly, as far as possible avoid  concealing Plumbing and drainage pipes, and all vertical downtake pipes should have min 4-6" offset from the valve and should be pr.tested properly.

I believe during construction, if proper water movement process is done and if not barriers blocks the water flow along with no unnecessary water drainage, water loss can be prevented to a lot of extent...

Cheapest way of water proofing are:

One should take pre-cautions during installation of Plumbing & Sanitory syatems.

It should be done by trained technicians.

Before installation proper planning is necessary.

All fittings should be branded and if possible it should be procurred from a single manufacturer.

All out-lets have proper dia suitable for quick discharge.

All horizontal lines and chambers in drainage system should have proper slop i.e. 1:100 

just watch and installing sanitation arrangements carefully and give  some distance between building and garden,and water pipes in walls install care fully,and for rising dampness treat on DPC must like some coatings bitumen and cement base,and also be carefull about neighbour buildings loose points of water falls. 

best of my knowledge bitumen with kerosine oil is the chepest form of waterprofing.and there should be proper slope. 

Indeed, mixture of Tar coal and Kerosene, and mixture of cow dung and Sand are very good option for waterproofing. But, if proper drainage system is provided during the whole construction process, then the necessity of additional prevention becomes very less.