You may find out a few methods at

http://www.ijer.ir/?_action=article&keywords=ecotoxicology

There is a lot of work using daphniids, particularly Daphnia magna (e.g. EU Guidance Document on aquatic ecotoxicology (2002), and recent studies have used freshwater crayfish innovatively, e.g. Iryna Kuklina in University of South Bohemia at Vodnany, Czech Republic, has used Astacus leptodactylus as an biomonitor.

Since the early 1970s the US Environmental Protection Agency Laboratory at Narragansett, Rhode Island has been engaged in ecotoxicological testing with invertebrates and has many hundreds of publications on the topic.  Their website is here: http://www2.epa.gov/research & you can learn a lot navigating their website   They began with single-chemical testing early on and moved on to testing photo-activated chemicals, synergetic chemical cocktails, and more recently on endocrine disruptors.   Staff Scientist Ronald Eisler has published many volumes on the early simple & inexpensive toxicological work with both marine invertebrates and vertebrates if you do a literature search on his name.

The problem of low-cost experimental ecotoxicology is that you don't monitore the substance concentrations in your test medium. Since most substances are either volatile, hydrolysable, photodegradable, poorly soluble, adsorbing..., it is very difficult to ascertain to what concentrations your organisms are exposed to if you don't monitore the actual concentrations.

See "principles of sound ecotoxicology" by Harris et al. (2014).

http://pubs.acs.org/doi/abs/10.1021/es4047507

I reckon that Neil Wang has covered my answer quite properly. There is no such thing as low-cost, this is, in my opinion, a underestimation of the biologist work. Let's suppose we can propose a method for any bioassay that is cheap (monetarily speaking), but how many biologist hours previously it demanded? This does not appear in the final price of what we call low-cost. Another aspect that I would like to add, low cost resembles low efficacy. We need robust experiments and a low cost strategy today, with an inapropiate decision may mean a high expediture in the future to restore the environemt, I believe you must be worried with your question first, and not to be worried with a low cost alternative.

Many thanks for all the useful information. When I ask about low-cost ecotoxicological methods I'm looking for methods that use cheap or even free software, videorecording, automatic monitoring, etc.  which is not incompatible with the analysis of actual concentrations in the bioassays (that it is usually not very expensive if they are conducted in external laboratories). A low-cost method can use a free software imagen analysis instead of a commercial one with the same (or even better) results, which means a big savings for a research project. The same can be said for video recording methods, etc. These aspects are very important in project with low budgets.

Again many thanks for your help and time

Álvaro

A Simple Low-Cost Field Mesocosm for Ecotoxicological Studies on Earthworms. 

PHYSIOL C PHARMACOL TOXICOL ENDOCRINOL , vol. 117, no. 1, pp. 31-40, 1997

DOI: 10.1016/S0742-8413(97)85596-X

GamTox: A Low-Cost Multimetric Ecotoxicity Test with Gammarus spp. for in and ex situ Application

International Journal of Zoology

Volume 2011 (2011), Article ID 574536, 7 pages