there are a lot of variables here as your aware but its a very good question.
so far i have built 2 trikes and im on my 3rd although technically iv built 3 and on my 4th but i dont count the 1st one as it was a very quick and rough build to see if i could sort of thing and on the build all i was working from was a couple of photos on the internet and i knew nothing about trike's in fact i had literately only heard of them a weak earlier lol. it took me about 8hrs of build time over 2 days and at a total cost of about £15 it ran ok but i got a shake at about 10mph that only got worse as i got faster so it lasted about a week before i took it apart and started on my first proper trike
as you can see it was a very rough build but it gave me a great starting point
then came what is affectionately known as MK1
as you can see i learnt a lot from my test build. also i had some info about how to build trike from this site and searching the web some of the info i got i still use today as reference. the best info iv found so far that i use more than any other is "The Recumbent Trike Design Primer Written By Rickey M. Horwitz" its a free pdf download from hell bent cycles and it has some very useful info for building and even buying a recumbent and finding what options are best for you.
MK1 took about 2 weeks to build that included 3 days for paint drying ect i was spending about 4 to 6 hrs a day apart from the paint drying days. and if i remember rightly it cost around £200 for parts that included all the metal welding rods new front wheels tyres and a couple of second hand bikes for headsets and rear wheel and brakes and gearing ect and of course the paint and so cheap panniers.
then there was MK2
MK2 again took about 2 weeks or so to build in fact im sure it took a little less build time as i had started to get my eye in and i had plans to follow rather than guess work. this build has been the most expensive so far but it also is the best so far i came in at around £600 but everything was new i even brought 2 brand new MTBs just to cut up for parts they were cheap budget end around £120 each and as you can see from the pics iv gone with the more expensive 14mm axled front wheels.
i also have the motorcycle panniers that i love as i can lock stuff up in them and dont have to worry to much about them i also included a 6 volt electrical system with key ignition that controlled the front and rear lights and also has 6 volt motorcycle indicators and i went with motorcycle mirrors to they were a bit more money but they dont vibrate or shake like cycle mirrors do at times also they looked better
im now on MK3 im taking this one slower just a few hours a week so far and the cost is still climbing im expecting it to be a bit more than MK2 as im getting the best i can afford also im building MK3 as a E-trike and its getting some smart extras.
iv tried to give you and idea of cost including everything like welding rods grinding and cutting disks ect
also having said all that a home built trike is never really finished on MK2 iv spent another £300 ish at least changing and updating things and adding an e kit.
i worked it out once and i reckon it would be possible to build a basic trike for around £100 possabley less depending on deals and what you can find to recycle. the cost comes down to two factors what you want included and what you can recycle. as for time it depends alot on how quick or slow you work, i know i can work quite quick and tend to plan what im going to do so i reckon if i had all the parts i could build a trike in under a week but thats just me lol
hope this has given you a rough idea
stuee