When I worked in the industry, we used Rospa for post-installation inspection of play equipment, advice that sort of thing. They have a useful collection of information leaflets which includes advice on wet pour, loose impact (bark, sand), grass.
Surfacing is horribly expensive and requires regular maintenance. Cost and maintenance varies from product to product so when comparing different types of surface, do look at the overall cost say over a 5 year period. Melcourt have information about bark mostly... bark needs regular maintenance and it degrades. However it can be suitable for CFH (Critical Fall Height) of up to 3.5 metres (depth of bark varies for different CFH) and isn't that complex to install - though on slope may be tricky depending on the gradient.
What is the CFH of the equipment you have? Is it 5 feet (about 1.5m)?
Fawns is a supplier who can supply various surfaces, including artificial grass (which should be cheaper than wetpour) and not need as much maintenance as loose fill.
What size of an area are you looking at? I would have thought that the most cost effective thing would be loose fill - recycled timber (Cushionfall), recycled rubber mulch (EcoScape), or play grade bark (playbark 10/50). It would need a base which could be constructed from timber edges and have a geo textile membrane (a dark coloured tiny mesh) which helps stop things growing through.
Delivery of loose fill material is cheapest via a lorry... which dumps a big of the stuff on your driveway!
Some types of loosefill can also be brought in bags but that costs more. EcoScape give some calculations for quantity needed... 24-32 kilos of rubber mulch per square metre, giving a depth of 75-100mm. A bag is about 10 kilos, so if you measure your space, you can calculate roughly how many bags.
Good grass is often perfectly fine for play equipment with CFH under 1.5m (1500mm). See this article about Grass as a safety surface.
What is Critical Fall Height? This article explains more about CFH. Put simply, it is the highest point of the play equipment that a child may fall from, measured from the child's feet to the ground below.