If you want a reliable motor with more than about 260rwkw, you should budget for AT LEAST ~$15k in parts, usually closer to $20k-$25k or more, as others have said above.
 
That's just the parts. If you do all the work yourself you will save a huge amount of money, but it will take much longer, and require experience and confidence to get it right.
If you get a decent workshop to do ALL of the work for you, the overall cost will roughly double due to labour costs.
 
In my case, I did about 10% of the work (eg. electrical stuff like wiring up the ECU), and a workshop did the rest, so the labour costs were very high.
I have always had a second car for daily driving, so it wasn't a big problem having my MR2 off the road. However, even with my MR2 at a workshop and being worked on fairly consistently, it still took about a year to be 'completed', plus an additional 4-6 months when I had to change some stuff and get an engineering certificate for all the mods.
 
If you purchased ALL required parts before you started the work (not likely), you would probably shave many months off the total completion time, however, it is very unlikely that you will go through the whole build process without changing your mind about some things.

if workshops are doing all the labour you need to add that to your cost. so far we just talked parts. figure in a couple of grand at least every time you have to drop the motor... so it's cheapest to do it in one go. 
 
my motors been out twice and i did it in stages and the bill is somewhere over $60k so far... but that includes a new gearbox from toyota for about $5k..... custom piping fabrication, a couple of tunes on the dyno etc. if you just bought available parts and did it in one hit you could save on some labour... 
 
I've stopped counting.

just a question, why do you want 400hp in an MR2?
 
 

What I'm trying to do is do small upgrades at a time, but make sure they'll still be compatible with my higher power goals. That way you don't have to spend too much at any one point, and it stays driveable for a longer period of time.
Ie I've upgraded the fuel pump and rail, which you can do just on their own and run like normal, but then you're ready to swap in bigger injectors when you get an ECU.
Have a water injection kit fitted. Planning on switching to a water to air intercooler, so will mount the heat exchanger and pump setup first before changing my cooler piping.
 
This is all irrelevant if you're doing a full ground up rebuild for massive power. But in that case I'd buy a spare block to build up separately and just drop in.

Because i just got it recently and i guess i want it be as quick as it looks because it looks damn fine. :D

Go GT3071 with supporting mods then rebuild the motor when it pops.

All this talk of cost is........lol

It'd make near the same power as a GT3076, but with slightly slower response than a comparable GT3071. More expensive than your run of the mill GT3071, but certainly an option if you want that little bit extra. To be more detailed with my recommendation:
 
Firstly, verify the condition of your motor (comp test etc). If ok, proceed to Phase 1 (join 1 & 2 if it's a tired motor to begin with). Make sure your gearbox synchros are still good too, since making lotsa power with a dodgy box is kinda silly. Make sure your radiator, coolant lines & everything else is good too (ie start with a good base).
 
Where you located? A few people here can recommend places (if Sydney, RaceSolutions in Penrith).
 
Phase 1:
GT3071 (or GTX) turbo (can use factory cast manifold with adaptor plate) - few sales in the States at the moment too
Cams (Kelford 264's come to mind) + matching valvesprings (mild porting of the cylinder head too if you can)
ARP head studs & MLS HG
Fuel system (pump, injectors, rail etc) - decide if you want/can go to E85/E-flex (dependant on availability in your area)
Intercooler - W2A OR trunk mount (don't bother with sidemounts if you want safe & sustainable power)
ECU - totally your choice here, but I'd recommend a Haltech Platinum (stay away from Wolf etc though - stick to what's proven to work)
Aftermarket intake manifold - RMR, RacerX etc (or custom)
Ignition - if running E85, you'd want stronger spark (M&W, NoddE's kit, MSD etc) - factory will be ok if running 98RON
Boost controller - again your preference here
CRW axle cages (or ATS)
Turbo back 3inch Exhaust (assume you already have one - but just in case)
 
Phase 2 (when engine dies or you have more money to burn):
Forged internals/bottom end
You have options here depending on block, displacement & compression ratio etc
For that turbo, you'll be ok with nice simple 3S block with 86.5mm pistons, 3S crank, Eagle rods etc (perfect rod ratio), but I'm sure if this day ever comes you'll post a thread/explore the options of a 2.1-2.3L motor with a 5S block.....lol