You will have to work cost effective. Even if you loose more than the enemy it's no problem if your units are cheaper.
I learned it is very effective to bring your boys in in waves (realy orky

). Just use 50 to 70% of your recources on the first army. Go engage and you will have casualties, but if you only loose the ceap units like shootaboys it's no problem. No you know what the enemy army looks like and bring out a second wave, that counters his army.
Also, here is a list of cost effective ork units that I use exclusively:
Infantery:
- shootas - good vs almost everything (below 95 armour) and good as a frontline meatshield
- big shootas - as a second line and for coverfire. Have at least one per army.
- warboss - just too have vision and keep up the morale of your bigger units. Try not to losse them and always cover with big shootas.
- tankbustas - apart from snipers and very ceap IG infatery the can deal with everything. If you get them in meele they do tons of damage and are your best titan killers.
- mega armoured nobs - in case you have a lot of money and there are chokepoints where you need more quality and quantity can't cut it
- stormboys - if you want to flank to get some snipers or artillery for an unseen angle.
- burna boys - if you need meele (very rare)
Everything but big shootas and warbosses will and should always take a huge amout of casualties and you have to bring in new ones constantly during the match. They need to be positioned to be a thread so that the more valuable units are save in the backlines.
Walkers:
- Deathdread (all meele) - if you need some heavy meele damage. They can kill a rus in one attack. Pretty expensive though. Best used in suprize attacks from the fog of war.
- squigoth with supa loba - as artillery units that boosts the morale (only one or 2 per army). Always keep your backlines at white morale (>90) to maximize damage output while your frontline meashield dies.
Vehicles:
- buggy with rokits - good as a scout and if you need something fast to go somewere. Pretty expensive though.
Tanks:
- skullhammer battlefortres - damage vs. lightly to medium armored units
- battlewagon with belly gun - damage vs. medium to heavy armored units
Artillery:
- get grots and put them in a battlewagon with supa cannon. most cost effective artellery you can get in the game. Go nutz. This is my main damage output apart from shootas and tankbustas.
- maybe some of the grot rokets but that's just if you plan on shooting highly armored targets
Airforce:
- NO
Titans:
- Gargants - pretty good damage for titans and other units with 6+ hp per kill.
All other units are not worht their cost compared to the ones listed above imo. Playing like that I won about 4 out of 5 games with orks.