According to Michael Mealling, the VP for Business Development at Masten Space Systems:
We have officially begun taking payload slot orders. Payloads are generally sold in 1 kilogram increments at $250 per kilo. For those that want a standardized form factor we are offering CanSat flights. A CanSat is the same size as a standard 12 oz soda can with a mass limit of 350 grams. You can buy kits from various vendors such as Pratt Hobbies. Normally CanSat flights are $199 but for a limited time they are going for an introductory $99.
Remember, this is suborbital only. We won’t be offering orbital payloads for several years and even then they won’t be anywhere near the $199 price.
350 gram CanSat - $99
1 kg Custom Payload - $250
2 kg Custom Payload - $500
5 kg Custom Payload - $1250
Over the next several months we will be releasing versions of the payload manual which includes interface definitions and physical characteristics such as vibration and G loads. Only customers who have signed up will receive these manuals so sign up early!