For around 12-13 day trip to Spiti valley for complete circuit and basic ameneties and public transport and homestays will cost you approx. 15000-18000 from Delhi and back. If short on budget, you can look for shared rides and shared rooms. Monasteries also offer rooms at very low prices. At least it would take 900/day/person if you spend your money very carefully and wisely.
Ladhak and Spiti are two different trips to plan out. seperate trips depends on factors like how many days you want to travel, stay food and transport, treks and others. so for ladhak 1 week plan would cost around 15k-20k and Spiti almost same prices.