lets put a few numbers - my estimate based on IGI is that there are some 180,000 recorded baptisms (I have some 154k in my database + a few more to gather); allowing about 4 or 5 children per marriage I would expect around 45k marriages - all these are for period 1600 to 1875. As a matter of comparison Brian has some 140k entries in his burial index. Thus we are looking at probably 2x as much as the Burial index.

Re Bishops Transcripts - my guess is less than 5000 entries needed.

Re illegitimates - allow 5% of baptisms 7.5 to 10k entries

Gathering info - mostly a correct retranscription of the 1911 would help, then correct this by referral to actual registers now that these are available on microfilm (some only at Manx Museum I think)

Ownership - a paper based copy is in my opinion an unviable option - the existing generation of computer illiterate genealogists are not going to be around for much longer, the cost of a complete paper set of burial, marriage and baptisms would on IoMFHS prices be well over 250 pounds. My version of Brians Burial index (ie about 20% of total) occupies 45Mbytes in a very searchable database (if done as per 1881 CD ROM would occupy maybe 5Mbytes) - thus complete set would fit on 1 CD ROM - duplication cost less than 50pence (little more if done in small batches) + very small postage.

Online - paid
(a) by subscription - possibly via FHS, IMHO not with current committee - see above comments
(b) private (as Brian's printed census indices) - yes but obviously need a structure to inherit data - I suspect corrections should tail off after a year or so thus mainly a clerical task
(c) free online - effectively paid by adverts - possibly, the storage and database access would not be that expensive. and might be able to sell idea to a well known manx website

I would be very willing to help in any way