If you already have the complete engine and can do the assembly work yourself it's not that pricey.
A complete piston kit is in the $450 - $500 range depending on what you buy and where; having the cylinder bored and finish honed on a four will be around $125 +/-. If you need to replace the bearings, need to have additional machine work done, et cetera the price starts adding up at a fairly steep rate. Stock conrods will cost around $85 each, Carillos $250 each... how fast do you want to go?
If you piston kit didn't include a headgasket you're looking at another forty bucks. As long as you don't tear up the other gaskets on the engine you can probably re-use the majority; budget a hundred bucks for gaskets and you should have money left over at the end.
Since the head is off, by all means have someone freshen up the valves... depending on how basic or how advanced you go on headwork this can cost anything from $150-ish for a basic three-angle labor-only job to several thousand for professional preparation.
If you tackle the re-assembly you're probably going to burn another couple of hundred bucks or so on specialized stuff (a pair of ring compressors, 2" micrometer to mike bearing journals, feeler gauges, a good torque wrench, Plastigauge, various gasket compounds, and so on).
If you're gonna let someone else turnkey the job, add $hundreds more and don't cut it cheap... care and precision in assembly are the keys to a strong-running engine that lasts.