COCOMO or FPA where (each FPA x avg market value)

Good question. Many seems to recommend COCOMO and FP. There quite number of interesting complexity metrics for web projects such as page complexity etc. There is a good book by Mendes, E (2008) cost estimation techniques for web projects, IGI and also other chapters by the same author. Hope this is useful.

In the following sites you can find information about.

http://en.wikipedia.org/wiki/COCOMO

http://csse.usc.edu/csse/research/COCOMOII/cocomo_main.html

http://galorath.com/?gclid=CPGt9szNwLcCFbMftAodlxsAJw

http://www.lyee-project.soft.iwate-pu.ac.jp/en/workplan/kb/Library/CostandEffortReferenceModels/Documents/Papers/K87.pdf

http://www.lyee-project.soft.iwate-pu.ac.jp/en/workplan/kb/Library/CostandEffortReferenceModels/Documents/Papers/BESW99.pdf

http://www.sigapp.org/acr/Issues/V8.2/Musilek.pdf

I strongly recommend taking a look at the research by prof. Emília Mendes from the Blekinge Institute of Technology in Sweden. She has several papers (probably more than 50) and two books on this topic ("Web Engineering" and "Cost Estimation Techniques for Web Projects"). I think that her best results were obtained using Bayesian Models built with within-company domain experts, she had a paper at last years ICSE on this, entitled "Using knowledge elicitation to improve Web effort estimation: Lessons from six industrial case studies". By the way, she is in ResearchGate, so you can check her publications and follow her updates. Hope that this helps and good luck!